Documentation: add dims v3 gateway API for python

Documentation/integrations.md

@@ -46,6 +46,7 @@
 - [cholcombe973/autodock](https://github.com/cholcombe973/autodock) - A docker deployment automation tool
 - [lisael/aioetcd](https://github.com/lisael/aioetcd) - (Python 3.4+) Asyncio coroutines client (Supports v2)
 - [txaio-etcd](https://github.com/crossbario/txaio-etcd) - Asynchronous etcd v3-only client library for Twisted (today) and asyncio (future)
+- [dims/etcd3-gateway](https://github.com/dims/etcd3-gateway) - etcd v3 API library using the HTTP grpc gateway
 
 **Node libraries**