Add a description of pod states in kubernetes.

Closes #1146 
I actually suggested that we get rid of terminated. Implementation-wise, failures on the machine, including OOMs, docker crashes, machine reboot, etc., would show up as Failed, and pods killed by a minion controller due to failure of minion health checks would be Stopped, since it would presumably delete those pods via the API. Other kinds of preemptions, evictions, replication controller downsizing, etc. would also appear as Stopped, as would user-initiated cancellation/abort of their pods.