drm_hwcomposer: Add README.md

Sean Paul requested to merge seanpaul/drm-hwcomposer:readme_v2 into master

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)

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