Provide some guidance on creating backups

Summary: A user asked for some instructions, so I wrote up some documentation.

Test Plan: Read document. This is more or less how secure.phabricator.com backups work and the one time we had a data loss issue restoration worked reasonably well.

Reviewers: btrahan, vrana

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D3825