2.16 release candidate 3 out

[Graham Percival]

I've released release candidate 3.  There always seems to be
confusion about this, so let's go over what this means:

- activity on master goes on as normal.

- nobody touches the release/unstable branch, than translators,
  who may merge with that if they want to and don't break
  anything.
  The question of whether translators have a stable branch or not
  is a separate matter and has nothing to do with the release
  plans.  It's just a question of how the translators want to
  organize themselves.

- when I say "nobody touches the release/unstable branch", I mean
  it.  There will be no new features, no ordinary bugfixes, no doc
  changes.

- if there are no Critical issues in two weeks, release/unstable
  becomes stable/2.16.

- if not, I make a new release/unstable based on master whenever
  those Critical issues are fixed.  This will obviously pick up
  any new features, bugfixes, or doc updates that happened in
  master.


The idea is to have 2.18 something like two months after 2.16, so
there is no point asking "can't you just wait until xyz is
finished".  There will always be another xyz, and that xyz will
always add some unintended critical bugs.  It is never a good idea
to delay a release until xyz is finished.

- Graham