This CL updates the README file accompanying the V4L2 components with
additional information about the V4L2 encoder. Examples are added to
show adding VP8 and VP9 encoders to the media_codecs_c2.xml file. A
description is added containing the list of supported codecs,
paramaters, input pixel formats and additional features.

BUG: 143333813
Test: None
Change-Id: I085076e67146ee3fb33864b3147db8fe091022b3