Writes documentation for Metro's API

Reviewed By: davidaurelio

Differential Revision: D6808245

fbshipit-source-id: 4c153c2c9dc8ddef3ebc564507281792267a2cc0

docs/API.md

@@ -9,8 +9,59 @@ title: API
 
 ---
 
+## Quick Start
+
+  - Compile a file
+
+    ```
+    await Metro.runBuild({
+      entry: 'index.js',
+      out: 'bundle.js',
+      config: Metro.loadMetroConfiguration(),
+    });
+    ```
+
+  - Run a server & watch the filesystem for changes
+
+    ```
+    await Metro.runServer({
+      port: 8080,
+      config: Metro.loadMetroConfiguration(),
+    });
+    ```
+
 ## Reference
 
-### `todo(fn)`
+All functions exposed below accept an additional `config` option. This object should be the [Metro configuration](CLI.md) exposed by your `metro.config.js` file - you can obtain it by simply requiring this file.
 
-*TODO*
+### `loadMetroConfiguration(filepath?: string, <options>)`
+
+**Basic options:** `cwd`, `basename`
+
+Load the Metro configuration, either from `filepath` if specified, or by traversing the directory hierarchy from `cwd` to the root until it finds a file named `basename` (by default `metro.config.js`). The returned configuration will have been normalized and merged with Metro's default values.
+
+### `findMetroConfiguration(filepath?: string, <options>)`
+
+**Basic options:** `cwd`, `basename`
+
+Same as above, but only locates the file.
+
+### `async runBuild(<options>)`
+
+**Required options:** `entry`, `out`
+
+**Basic options:** `dev`, `optimize`, `platform`, `sourceMap`, `sourceMapUrl`
+
+Bundles `entry` for the given `platform`, and saves it to location `out`. If `sourceMap` is set, also generates a source map. The source map will be inlined, unless `sourceMapUrl` is also defined. In the latter case, a new file will be generated with the basename of the `sourceMapUrl` parameter
+
+### `async runServer(<options>)`
+
+**Basic options:** `host`, `port`, `secure`, `secureKey`, `secureCert`, `hmrEnabled`
+
+Starts a full Metro HTTP server. It will listen on the specified `host:port`, and can then be queried to retrieve bundles for various entry points. If the `secure` family of options are present, the server will be exposed over HTTPS. If `hmrEnabled` is set, the server will also expose a websocket server and inject the HMR client into the generated bundles.
+
+### `createConnectMiddleware(<options>)`
+
+**Basic options:** `port`
+
+Instead of creating the full server, creates a Connect middleware that answers to bundle requests. This middleware can then be plugged into your own servers. The `port` parameter is optional and only used for logging purposes.