docs(contrib/rackspace): add manual update instructions

Author: carmstrong

contrib/rackspace/README.md

@@ -60,6 +60,16 @@ By default, the Makefile will provision 1 router. You can override this by setti
 $ export DEIS_NUM_ROUTERS=2
 ```
 
+### 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+Due to image publishing limitations on Rackspace, CoreOS images on Rackspace are frequently out of date.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

@@ -48,3 +48,4 @@ 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."