Add info on in-band text tracks

Describe ID3 metadata parsing and translation to metadata text tracks. Link to helpful background docs.

@@ -155,6 +155,20 @@ the actual rendering quality change does not occur simultaneously with
 this event; a new segment must be requested and the existing buffer
 depleted first.
 
+### In-Band Metadata
+The HLS tech supports [timed
+metadata](https://developer.apple.com/library/ios/#documentation/AudioVideo/Conceptual/HTTP_Live_Streaming_Metadata_Spec/Introduction/Introduction.html)
+embedded as [ID3 tags](http://id3.org/id3v2.3.0). When a stream is
+encountered with embedded metadata, an [in-band metadata text
+track](https://html.spec.whatwg.org/multipage/embedded-content.html#text-track-in-band-metadata-track-dispatch-type)
+will automatically be created and populated with cues as they are
+encountered in the stream. Only UTF-8 encoded
+[TXXX](http://id3.org/id3v2.3.0#User_defined_text_information_frame)
+and [WXXX](http://id3.org/id3v2.3.0#User_defined_URL_link_frame) ID3
+frames are currently mapped to cue points. There are lots of guides
+and references to using text tracks [around the
+web](http://www.html5rocks.com/en/tutorials/track/basics/).
+
 ### Testing
 
 For testing, you can either run `npm test` or use `grunt` directly.