libraries-and-tools.md: add C++ client package

Documentation/libraries-and-tools.md

@@ -63,6 +63,7 @@
 - [suryanathan/etcdcpp](https://github.com/suryanathan/etcdcpp) - Supports v2 (with waits)
 - [nokia/etcd-cpp-api](https://github.com/nokia/etcd-cpp-api) - Supports v2
 - [nokia/etcd-cpp-apiv3](https://github.com/nokia/etcd-cpp-apiv3) - Supports v3
+- [plasticbox/etcdclient](https://github.com/plasticbox/etcdclient) - Supports v3
 
 **Clojure libraries**