R1:6cfbf481a5f6

Provide more detailed guidance on upgrading Phabricator and the "stable" branch

Summary: Maintaining "stable" is worthwhile now that we're running the cluster, so document its existence.

Test Plan: Reading.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D13446