drm_hwcomposer: Add README.md
Although we had CONTRIBUTING.md, gitlab didn't really highlight it. So I tried putting this info in the wiki, but that was equally hard to discover. then I thought gitlab pages was the right way to go, but no.
Turns out README.md will be harvested by gitlab and shown on the project's frontpage \o/
Signed-off-by: Sean Paul seanpaul@chromium.org
Fixes #9 (closed)