More info on using the RAPI

Add some additional information about how to use/test the RAPI,
including how to specify authentication credentials.

Cherry picked from 681f4f7b on master.

Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>