Added API to README

Author: jforbes

README.md

@@ -401,6 +401,12 @@ The property defaults to `false`.
 
 With `customTagParsers` you can pass an array of custom m3u8 tag parser objects. See https://github.com/videojs/m3u8-parser#custom-parsers
 
+##### customTagMappers
+* Type: `Array`
+* can be used as a source option
+
+Similar to `customTagParsers`, with `customTagMappers` you can pass an array of custom m3u8 tag mapper objects. See https://github.com/videojs/m3u8-parser#custom-parsers
+
 ### Runtime Properties
 Runtime properties are attached to the tech object when HLS is in
 use. You can get a reference to the HLS source handler like this: