Added a CONTRIBUTING.md file.
Added a CONTRIBUTING.md file.

This file is intended to be a short manual describing the repository
structure and classical tasks.

BUG=
R=agable@chromium.org, hinoka@google.com

Review URL: https://codereview.chromium.org/359543002

nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra