doc: add backword_compatibility.md

@barakmich @jonboulle @philips @kelseyhightower 
I was going to add something here, like "To better explain which machines are part of an etcd cluster, and to simplify the keyspace for all your use cases, we've unified `v2/admin` and `v2/keys/_etcd` under the new [v2/member API][memberapi]