Add list of load error codes from the spec to docs

Fixes #1370

README.md

@@ -256,6 +256,12 @@ new Howl({
 Fires when the sound is loaded.
 #### onloaderror `Function`
 Fires when the sound is unable to load. The first parameter is the ID of the sound (if it exists) and the second is the error message/code.
+
+The load error codes are [defined in the spec](http://dev.w3.org/html5/spec-author-view/spec.html#mediaerror):
+**1** - The fetching process for the media resource was aborted by the user agent at the user's request.
+**2** - A network error of some description caused the user agent to stop fetching the media resource, after the resource was established to be usable.
+**3** - An error of some description occured while decoding the media resource, after the resource was established to be usable.
+**4** - The media resource indicated by the src attribute or assigned media provider object was not suitable.
 #### onplayerror `Function`
 Fires when the sound is unable to play. The first parameter is the ID of the sound and the second is the error message/code.
 #### onplay `Function`