Merge remote-tracking branch 'remotes/cohuck/tags/stable-doc-20180219' into staging

Add some doc for the stable process.

# gpg: Signature made Mon 19 Feb 2018 10:01:19 GMT
# gpg:                using RSA key DECF6B93C6F02FAF
# gpg: Good signature from "Cornelia Huck <conny@cornelia-huck.de>"
# gpg:                 aka "Cornelia Huck <huckc@linux.vnet.ibm.com>"
# gpg:                 aka "Cornelia Huck <cornelia.huck@de.ibm.com>"
# gpg:                 aka "Cornelia Huck <cohuck@kernel.org>"
# gpg:                 aka "Cornelia Huck <cohuck@redhat.com>"
# Primary key fingerprint: C3D0 D66D C362 4FF6 A8C0  18CE DECF 6B93 C6F0 2FAF

* remotes/cohuck/tags/stable-doc-20180219:
  docs: document our stable process

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>