doc: add doc for proxy

Fix #1605
We say what this doesn't do well, but shouldn't we also say what you gain from running the proxy? My rewrite:

> etcd can run as a transparent proxy, where the local endpoint acts as a reverse proxy and forwards client requests to an active etcd cluster.

> Running etcd as a proxy allows for easily discovery of etcd within your infrastructure, since it can run on each machine at `127.0.0.1:4001` without decreasing write performance of the cluster.

> When running as a transparent proxy, etcd still keeps track of the cluster leader and followers to ensure that traffic is always forwarded to the most appropriate location. This activity is limited to simple routing – etcd proxies don't increase the resiliency of your cluster or participate in the consensus process.

Is "endpoint" the right word to use?