Move DEPS file docs into a README.md

Moving the comments out into a separate Markdown file makes this info
more easily discoverable as well as indexable by search engines.

Previously, it wasn't clear where to find documentation on the DEPS
file format, as there are no entries in Chrome's //docs directory and
Google turns up no results.

BUG=none
R=brettw@chromium.org

Review-Url: https://codereview.chromium.org/2799113002 .
4 files changed