add documentation for etcd storage
add documentation for etcd storage

This adds references to etcd storage, including:
- only supports etcd v3
- list of options and their meanings when connecting to etcd cluster

wolfeidau/dex,rithujohn191/dex,ericchiang/dex,coreos/dex,wolfeidau/dex,ericchiang/dex,rithujohn191/dex,coreos/dex,coreos/dex,wolfeidau/dex,rithujohn191/dex,ericchiang/dex