[add] Image 'prefetch' and 'getSize' statics

Fix #160

docs/components/Image.md

@@ -75,6 +75,23 @@ Example usage:
 <Image resizeMode={Image.resizeMode.contain} />
 ```
 
+## Methods
+
+static **getSize**(uri: string, success: (width, height) => {}, failure: function)
+
+Retrieve the width and height (in pixels) of an image prior to displaying it.
+This method can fail if the image cannot be found, or fails to download.
+
+(In order to retrieve the image dimensions, the image may first need to be
+loaded or downloaded, after which it will be cached. This means that in
+principle you could use this method to preload images, however it is not
+optimized for that purpose, and may in future be implemented in a way that does
+not fully load/download the image data.)
+
+static **prefetch**(url: string): Promise
+
+Prefetches a remote image for later use by downloading it.
+
 ## Examples
 
 ```js