docs(configuration): add cluster configuration

#849
OK. What about:

Only applied if the etcd instance is the first peer in the cluster.

I think we also need a section of this that talks about and links to the cluster config API similar to my other PR: https://github.com/coreos/etcd/pull/850/files#diff-c2e93b5d997618cfe14f6ea88f78f0d3R15