[admob][android] Start implementation of Rewarded Video

docs/modules/admob.md

@@ -4,10 +4,66 @@ The admob allows you to display adverts in your app, using your account from [Ad
 
 RNFirebase allows you to display Banners, Interstitials, Native Ads & Rewarded Videos.
 
-## Banner
+### Banner
 
-## Interstitial
+AdMob Banners in RNFirebase are exported as a usable React component, allowing you to integrate it easily into your existing app very easily.
 
-## Native
+```js
+const Banner = firebase.admob.Banner;
+...
+render() {
+  return (
+    <Banner
+
+    />
+  );
+}
+
+```
+
+### Interstitial
+
+An interstitial is a full screen advert which creates a new activity on top of React. As they need to be controlled,
+allowing the developer to choose when to display them they're not available as a component. Instead they're controlled via
+method calls.
+
+To request an interstitial from AdMob, the `loadAd` method must be called with an instance of `AdRequest` (see below for full API):
+
+```js
+const advert = firebase.admob().interstitial('ca-app-pub-3940256099942544/1033173712');
+
+const AdRequest = firebase.admob.AdRequest;
+const request = new AdRequest();
+request.addKeyword('foo').addKeyword('bar');
+
+// Load the advert with our AdRequest
+advert.loadAd(request.build());
+
+// Simulate the interstitial being shown "sometime" later during the apps lifecycle
+setTimeout(() => {
+  if (advert.isLoaded()) {
+    advert.show();
+  } else {
+    // Unable to show interstitial - not loaded yet.
+  }
+}, 1000);
+
+```
+
+### Native
+
+### Rewarded Video
+
+## Statics
+
+### Banner
+> Accessed via `firebase.admob.Banner`.
+
+Exports a React component with the following PropTypes:
+
+
+### AdRequest
+> Accessed via `firebase.admob.AdRequest`.
+
+Used to build a request object to pass into AdMob requests. Exposes the following chainable methods:
 
-## Rewarded Video