MLBEDSW-3380 Update readme with build flags

mlw_codec is part of the codebase and has build flags.
README has been updated to include these.

Also, added -Werror to the list, as we must build without any warnings,
so treat warnings as errors.

Signed-off-by: Michael McGeagh <michael.mcgeagh@arm.com>
Change-Id: I10114bb013fad1ec1685fafc2e41c18ff12d9f9d