docs: document the staging branch and add reference to it

docs: document the staging branch and add reference to it

A while back we agreed that having a live/staging branch is beneficial.
Sadly we forgot to document that, so here is my first attempt.

Document the caveat that the branch history is not stable.

CC: Andres Gomez <agomez@igalia.com>
CC: Dylan Baker <dylan@pnwbakers.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Juan A. Suarez <jasuarez@igalia.com>