docs(contrib/rackspace): document manual update for CoreOS

Since the Rackspace images are updated infrequently, we still use
CoreOS 317.0.0 images on Rackspace. This commit details the workaround
to manually update CoreOS to the latest release from the channel.

Author: carmstrong

contrib/rackspace/README.md

@@ -53,6 +53,16 @@ For more information, see [optimal etcd cluster size](https://github.com/coreos/
 
 Deis clusters of less than 3 nodes are unsupported.
 
+### Update CoreOS
+Due to image publishing limitations on Rackspace, CoreOS images on Rackspace are frequently out of date.
+Each machine needs to be updated. On each one, run:
+
+```console
+$ sudo systemctl unmask update-engine && sudo systemctl start update-engine && sudo update_engine_client -update && sudo reboot
+```
+
+Once the machine is rebooted, it should have a recent CoreOS version.
+
 ### Initialize the cluster
 Once the cluster is up, get the hostname of any of the machines from Rackspace, set
 FLEETCTL_TUNNEL, and issue a `make run` from the project root:

contrib/rackspace/provision-rackspace-cluster.sh

@@ -45,3 +45,4 @@ done
 
 echo_green "Your Deis cluster has successfully deployed to Rackspace."
 echo_green "Please continue to follow the instructions in the README."
+echo_yellow "NOTE: For this release, you must also update CoreOS on these machines. See the README for details."