Document and remove some scary warnings from repository hosting

Summary: Fixes T2230. This isn't a total walk in the park to configure, but should work for early adopters now.

Test Plan: Read documentation, browsed UI.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2230

Differential Revision: https://secure.phabricator.com/D7634