GitLab

Migrate Contributing to Chromium documentation to Markdown.

Change-Id: I9e010bc19bb4dd99c7725c2a17559eff43634238
Reviewed-on: https://chromium-review.googlesource.com/c/1488477Reviewed-by: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635911}