Added defaultSource prop documentation to README

Author: IjzerenHein

README.md

@@ -75,6 +75,7 @@ export class MyFirebaseImageView extends Component<void, void, void> {
       <ImageView
         {...imageProps}
         path='firebase/storage/path'
+        defaultSource={require('./placeholder.png')} // optional, show placeholder until image is loaded
         timestamp={0} //optional, can be used to specify last modified time for same storage path
         resizeMode='cover' //'cover', 'contain', 'stretch'
       />
@@ -95,10 +96,13 @@ export class MyFirebasePhotoView extends Component<void, void, void> {
       <PhotoView
         {...imageProps}
         path='firebase/storage/path'
+        defaultSource={require('./placeholder.png')} // optional, show placeholder until image is loaded
         timestamp={0} //optional, can be used to specify last modified time for same storage path
         resizeMode='cover' //'cover', 'contain', 'stretch'
       />
     )
   }
 }
 ```
+
+> Note: On Android, the `defaultSource` prop is ignored on debug builds.