Updated Hello Mobile Navigation for clarity (#265)

Per request [#228](https://github.com/react-community/react-navigation/issues/228), I've updated the the Hello Mobile Navigation guide to try and make it more approachable to newcomers like myself. I changed the order of some of the explanations and broke out passing params into its own section. This should make the guide easier to follow as only one new concept is introduced per section; a common principle in teaching methodology.

I'm open to suggestions as I am also a newcomer to both React Native and React Navigation. So it's very possible that there are concepts that I've misunderstood or am not explaining well.

docs/guides/Guide-Intro.md

@@ -32,7 +32,7 @@ Now lets create the new file for our app implementation, `App.js`.
 
 ## Introducing Stack Navigator
 
-For our app, we want to use the `StackNavigator` because we want a conceptual 'stack' navigation, where each new screen is put on the top of the stack, and going back removes a screen from the top of the stack. Lets start with just one screen:
+For our app, we want to use the `StackNavigator` because we want a conceptual 'stack' navigation, where each new screen is put on the top of the stack and going back removes a screen from the top of the stack. Let's start with just one screen:
 
 ```js
 import React from 'react';
@@ -68,7 +68,65 @@ first-screen
 
 ## Adding a New Screen
 
-Lets create a button in the `HomeScreen` component that links to our second page:
+In our `App.js` file, let's add a new screen called `ChatScreen`:
+
+```js
+class ChatScreen extends React.Component {
+  static navigationOptions = {
+    title: 'Chat with Lucy',
+  };
+  render() {
+    return (
+      <View>
+        <Text>Chat with Lucy</Text>
+      </View>
+    );
+  }
+}
+```
+
+We can then add a button to our `HomeScreen` component that links to `ChatScreen` using the `routeName` `Chat`.
+
+```js
+class HomeScreen extends React.Component {
+  static navigationOptions = {
+    title: 'Welcome',
+  };
+  render() {
+    const { navigate } = this.props.navigation;
+    return (
+      <View>
+        <Text>Hello, Chat App!</Text>
+        <Button
+          onPress={() => navigate('Chat')}
+          title="Chat with Lucy"
+        />
+      </View>
+    );
+  }
+}
+```
+
+We're using the navigate function from the [screen navigation prop](/docs/navigators/navigation-prop) to go to `ChatScreen`. But that won't work until we add this to our `StackNavigator` like so:
+
+```js
+const SimpleApp = StackNavigator({
+  Home: { screen: HomeScreen },
+  Chat: { screen: ChatScreen },
+});
+```
+
+Now you can navigate to your new screen, and go back:
+
+```phone-example
+first-navigation
+```
+
+## Passing params
+
+Hardcoding a name into the `ChatScreen` isn't ideal. It'd be more useful if we could pass a name to be rendered instead, so let's do that.
+
+In addition to specifying the target `routeName` in the navigate function, we can pass params that will be put into the new route. First, we'll edit our `HomeScreen` component to pass a `name` param into the route.
 
 ```js
 class HomeScreen extends React.Component {
@@ -90,9 +148,7 @@ class HomeScreen extends React.Component {
 }
 ```
 
-We're using the navigate function from the [screen navigation prop](/docs/navigators/navigation-prop). In addition to specifying the target `routeName`, we can also pass params that will be put into the new route.
-
-Now let's create the Chat screen that displays the `name` param passed in through the route:
+We can then edit our `ChatScreen` component to display the `name` param that was passed in through the route:
 
 ```js
 class ChatScreen extends React.Component {
@@ -110,14 +166,9 @@ class ChatScreen extends React.Component {
     );
   }
 }
-
-const SimpleApp = StackNavigator({
-  Home: { screen: HomeScreen },
-  Chat: { screen: ChatScreen },
-});
 ```
 
-Now you can navigate to your new screen, and go back:
+Now you can see the name when you navigate to the Chat screen. Try changing the `name` param in `HomeScreen` and see what happens!
 
 ```phone-example
 first-navigation