v4l2_codec2 encoder: Add encoder-specific documentation.

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