meta-google: add maintainers file

Add a maintainers file for the meta-google layer.

(From meta-google rev: 91a5446f9f0e176874f585c04261c8beaf1d5ab9)

Change-Id: Ifea3727d5b1a86902430482908a4b3bd7a2fab83
Signed-off-by: Patrick Venture <venture@google.com>
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>