Update Features section on etcd.io main page

The`Features` content on etcd.io points to V2 specific features that needs to be updated. e.g. it says ` etcd store data in hierarchically organized directories ` Also the interface should add gRPC.

cc @gyuho @jingyih @ptabor

@nate-double-u @chalin I will work on it or help with it there is any related updates in progress. Thanks!