docs: add official client to libraries-and-tools

Documentation/libraries-and-tools.md

@@ -13,7 +13,8 @@
 
 **Go libraries**
 
-- [go-etcd](https://github.com/coreos/go-etcd) - Supports v2
+- [etcd/client](https://github.com/coreos/etcd/blob/master/client) - the officially maintained Go client
+- [go-etcd](https://github.com/coreos/go-etcd) - the deprecated official client. May be useful for older (<2.0.0) versions of etcd.
 
 **Java libraries**