docs: document our stable process

Some pointers on how to get a patch into stable.

[contains some suggestions by mdroth and eblake]
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>

docs/devel/stable-process.rst

+QEMU and the stable process
+===========================
+
+QEMU stable releases
+--------------------
+
+QEMU stable releases are based upon the last released QEMU version
+and marked by an additional version number, e.g. 2.10.1. Occasionally,
+a four-number version is released, if a single urgent fix needs to go
+on top.
+
+Usually, stable releases are only provided for the last major QEMU
+release. For example, when QEMU 2.11.0 is released, 2.11.x or 2.11.x.y
+stable releases are produced only until QEMU 2.12.0 is released, at
+which point the stable process moves to producing 2.12.x/2.12.x.y releases.
+
+What should go into a stable release?
+-------------------------------------
+
+Generally, the following patches are considered stable material:
+- Patches that fix severe issues, like fixes for CVEs
+- Patches that fix regressions
+
+If you think the patch would be important for users of the current release
+(or for a distribution picking fixes), it is usually a good candidate
+for stable.
+
+
+How to get a patch into QEMU stable
+-----------------------------------
+
+There are various ways to get a patch into stable:
+
+* Preferred: Make sure that the stable maintainers are on copy when you send
+  the patch by adding
+
+  .. code::
+
+     Cc: qemu-stable@nongnu.org
+
+  to the patch description. By default, this will send a copy of the patch
+  to ``qemu-stable@nongnu.org`` if you use git send-email, which is where
+  patches that are stable candidates are tracked by the maintainers.
+
+* You can also reply to a patch and put ``qemu-stable@nongnu.org`` on copy
+  directly in your mail client if you think a previously submitted patch
+  should be considered for a stable release.
+
+* If a maintainer judges the patch appropriate for stable later on (or you
+  notify them), they will add the same line to the patch, meaning that
+  the stable maintainers will be on copy on the maintainer's pull request.
+
+* If you judge an already merged patch suitable for stable, send a mail
+  (preferably as a reply to the most recent patch submission) to
+  ``qemu-stable@nongnu.org`` along with ``qemu-devel@nongnu.org`` and
+  appropriate other people (like the patch author or the relevant maintainer)
+  on copy.
+
+Stable release process
+----------------------
+
+When the stable maintainers prepare a new stable release, they will prepare
+a git branch with a release candidate and send the patches out to
+``qemu-devel@nongnu.org`` for review. If any of your patches are included,
+please verify that they look fine, especially if the maintainer had to tweak
+the patch as part of back-porting things across branches. You may also
+nominate other patches that you think are suitable for inclusion. After
+review is complete (may involve more release candidates), a new stable release
+is made available.