Added the Mux Container Spec (RIFF structure) for the features supported
(Color Profile, XMP Metadata, Animation & Tiling) via WebP Mux. Also
added README file with example psudo code for using the Mux API(s) and a
short description on webpmux (comand line tool, planned to be pushed out
in subsequent Git change).
Change-Id: I87c322cada89955bd758dd524a862a8cbc407541
