R1:3f3626c11a04

Write some documentation about Drydock security and repository automation

Summary: Ref T182. Ref T9519. Some of what this describes doesn't exist yet, but should soon.

Test Plan: Read documentation.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T182, T9519

Differential Revision: https://secure.phabricator.com/D14258