Add content from Sites to Atom guide.

This CL adds the relevant content at
https://www.chromium.org/developers/using-atom-as-your-ide to the Atom
guide at docs/atom.md, so the Sites page can be replaced with a link to
the Markdown guide.

BUG=

Review-Url: https://codereview.chromium.org/2557613002
Cr-Commit-Position: refs/heads/master@{#436497}
1 file changed