Add docs pages for basics: Dimensions and Layout

Summary:
These pages should sufficiently give a beginner enough information to make most layouts in React Native. They should go after the basics-style page, whenever that is ready.

Having a single page for Layout was too much, so I split it into two: Dimensions and Layout.

![dimensions react native a framework for building native apps using react](https://cloud.githubusercontent.com/assets/1198882/16311045/c6918b64-3923-11e6-8cc9-daeda9eb40e6.png)

![layout react native a framework for building native apps using react](https://cloud.githubusercontent.com/assets/1198882/16310233/9a66405a-3920-11e6-9ef6-1594f7228e83.png)

lacker
Closes https://github.com/facebook/react-native/pull/8364

Differential Revision: D3477147

Pulled By: lacker

fbshipit-source-id: 1ef31ac0a64e43166a7581b38fa8263282672eeb

docs/Basics-Component-ListView.md

@@ -4,7 +4,7 @@ title: ListView
 layout: docs
 category: The Basics
 permalink: docs/basics-component-listview.html
-next: basics-network
+next: basics-dimensions
 ---
 
 On mobile devices, lists are a core element in many applications. The [`ListView`](/react-native/docs/listview.html#content) component is a special type of [`View`](/react-native/docs/basics-component-view.html) that displays a *vertically* scrolling list of changing, but similarly structured, data.

docs/Basics-Dimensions.md

@@ -0,0 +1,63 @@
+---
+id: basics-dimensions
+title: Dimensions
+layout: docs
+category: The Basics
+permalink: docs/basics-dimensions.html
+next: basics-layout
+---
+
+A component's dimensions determine its size on the screen.
+
+#### Fixed Dimensions
+
+The simplest way to set the dimensions of a component is by adding a fixed `width` and `height` to style. All dimensions in React Native are unitless, and represent density-independent pixels.
+
+```ReactNativeWebPlayer
+import React from 'react';
+import { AppRegistry, View } from 'react-native';
+
+class AwesomeProject {
+  render() {
+    return (
+      <View>
+        <View style={{width: 50, height: 50, backgroundColor: 'powderblue'}} />
+        <View style={{width: 100, height: 100, backgroundColor: 'skyblue'}} />
+        <View style={{width: 150, height: 150, backgroundColor: 'steelblue'}} />
+      </View>
+    );
+  }
+};
+
+AppRegistry.registerComponent('AwesomeProject', () => AwesomeProject);
+```
+
+Setting dimensions this way is common for components that should always render at exactly the same size, regardless of screen dimensions.
+
+#### Flex Dimensions
+
+Use `flex` in a component's style to have the component expand and shrink dynamically based on available space. Normally you will use `flex: 1`, which tells a component to fill all available space, shared evenly amongst each other component with the same parent. The larger the `flex` given, the higher the ratio of space a component will take compared to its siblings.
+
+> A component can only expand to fill available space if its parent has dimensions greater than 0. If a parent does not have either a fixed `width` and `height` or `flex`, the parent will have dimensions of 0 and the `flex` children will not be visible.
+
+```ReactNativeWebPlayer
+import React from 'react';
+import { AppRegistry, View } from 'react-native';
+
+class AwesomeProject {
+  render() {
+    return (
+      // Try removing the `flex: 1` on the parent View.
+      // The parent will not have dimensions, so the children can't expand.
+      // What if you add `height: 300` instead of `flex: 1`?
+      <View style={{flex: 1}}>
+        <View style={{flex: 1, backgroundColor: 'powderblue'}} />
+        <View style={{flex: 2, backgroundColor: 'skyblue'}} />
+        <View style={{flex: 3, backgroundColor: 'steelblue'}} />
+      </View>
+    );
+  }
+};
+
+AppRegistry.registerComponent('AwesomeProject', () => AwesomeProject);
+```

docs/Basics-Layout.md

@@ -0,0 +1,104 @@
+---
+id: basics-layout
+title: Layout
+layout: docs
+category: The Basics
+permalink: docs/basics-layout.html
+next: basics-network
+---
+
+A component can specify the layout of its children using the flexbox algorithm. Flexbox is designed to provide a consistent layout on different screen sizes.
+
+You will normally use a combination of `flexDirection`, `alignItems`, and `justifyContent` to achieve the right layout.
+
+> Flexbox works the same way in React Native as it does in CSS on the web, with a few exceptions. The most notable one: the defaults are different, with `flexDirection`  defaulting to `column` instead of `row`, and `alignItems` defaulting to `stretch` instead of `flex-start`.
+
+#### Flex Direction
+
+Adding `flexDirection` to a component's `style` determines the **primary axis** of its layout. Should the children be organized horizontally (`row`) or vertically (`column`)? The default is `column`.
+
+```ReactNativeWebPlayer
+import React from 'react';
+import { AppRegistry, View } from 'react-native';
+
+class AwesomeProject {
+  render() {
+    return (
+      // Try setting `flexDirection` to `column`.
+      <View style={{flex: 1, flexDirection: 'row'}}>
+        <View style={{width: 50, height: 50, backgroundColor: 'powderblue'}} />
+        <View style={{width: 50, height: 50, backgroundColor: 'skyblue'}} />
+        <View style={{width: 50, height: 50, backgroundColor: 'steelblue'}} />
+      </View>
+    );
+  }
+};
+
+AppRegistry.registerComponent('AwesomeProject', () => AwesomeProject);
+```
+
+#### Justify Content
+
+Adding `justifyContent` to a component's style determines the **distribution** of children along the **primary axis**. Should children be distributed at the start, the center, the end, or spaced evenly? Available options are `flex-start`, `center`, `flex-end`, `space-around`, and `space-between`.
+
+```ReactNativeWebPlayer
+import React from 'react';
+import { AppRegistry, View } from 'react-native';
+
+class AwesomeProject {
+  render() {
+    return (
+      // Try setting `justifyContent` to `center`.
+      // Try setting `flexDirection` to `row`.
+      <View style={{
+        flex: 1,
+        flexDirection: 'column',
+        justifyContent: 'space-between',
+      }}>
+        <View style={{width: 50, height: 50, backgroundColor: 'powderblue'}} />
+        <View style={{width: 50, height: 50, backgroundColor: 'skyblue'}} />
+        <View style={{width: 50, height: 50, backgroundColor: 'steelblue'}} />
+      </View>
+    );
+  }
+};
+
+AppRegistry.registerComponent('AwesomeProject', () => AwesomeProject);
+```
+
+#### Align Items
+
+Adding `alignItems` to a component's style determines the **alignment** of children along the **secondary axis** (if the primary axis is `row`, then the secondary is `column`, and vice versa). Should children be aligned at the start, the center, the end, or stretched to fill? Available options are `flex-start`, `center`, `flex-end`, and `stretch`.
+
+> For `stretch` to have an effect, children must not have a fixed dimension along the secondary axis. In the following example, setting `alignItems: stretch` does nothing until the `width: 50` is removed from the children.
+
+```ReactNativeWebPlayer
+import React from 'react';
+import { AppRegistry, View } from 'react-native';
+
+class AwesomeProject {
+  render() {
+    return (
+      // Try setting `alignItems` to 'flex-start'
+      // Try setting `justifyContent` to `flex-end`.
+      // Try setting `flexDirection` to `row`.
+      <View style={{
+        flex: 1,
+        flexDirection: 'column',
+        justifyContent: 'center',
+        alignItems: 'center',
+      }}>
+        <View style={{width: 50, height: 50, backgroundColor: 'powderblue'}} />
+        <View style={{width: 50, height: 50, backgroundColor: 'skyblue'}} />
+        <View style={{width: 50, height: 50, backgroundColor: 'steelblue'}} />
+      </View>
+    );
+  }
+};
+
+AppRegistry.registerComponent('AwesomeProject', () => AwesomeProject);
+```
+
+#### API Reference
+
+We've covered the basics, but there are many other styles you may need for layouts. The full list is available [here](./docs/flexbox.html).