Adding supported formats to Image component docs

Summary:
The lack of very clear docs on supported image formats is causing unnecessary issues (#13806).
This should mitigate and keep the issue list cleaner.

Its only a documentation change. No code changes made.
Closes https://github.com/facebook/react-native/pull/13807

Differential Revision: D5023002

Pulled By: javache

fbshipit-source-id: b1e3562ae7649cf71833b300e125966d6e832a69

Libraries/Image/Image.ios.js

@@ -142,6 +142,9 @@ const Image = React.createClass({
      * The native side will then choose the best `uri` to display based on the
      * measured size of the image container. A `cache` property can be added to
      * control how networked request interacts with the local cache.
+     *
+     * The currently supported formats are `png`, `jpg`, `jpeg`, `bmp`, `gif`,
+     * `webp` (Android only), `psd` (iOS only).
      */
     source: ImageSourcePropType,
     /**