docs(google-plus): add basic docs

src/plugins/google-plus.ts

@@ -7,6 +7,9 @@ import { Cordova, Plugin } from './plugin';
  * ```typescript
  * import { GooglePlus } from 'ionic-native';
  *
+ * GooglePlus.login()
+ *   .then(res => console.log(res))
+ *   .catch(err => console.error(err));
  *
  * ```
  */