Documentation: added ETCD Manager tool

New tool: ETCD Manager

ETCD Manager is a multi-platform ETCD v3 client. Currently, builds are available form Mac, Wiindows and Linux, but iOS / Android builds will also be added in the future.
It aims to be a modern, efficient and easy to use GUI with full coverage of ETCD APIs / functionality. The first public (beta) release is already available.

Documentation/integrations.md

@@ -18,6 +18,7 @@ title: Libraries and tools
 - [etcdsh](https://github.com/kamilhark/etcdsh) - A command line client with support of command history and tab completion. Supports v2
 - [etcdloadtest](https://github.com/sinsharat/etcdloadtest) - A command line load test client for etcd version 3.0 and above.
 - [lucas](https://github.com/ringtail/lucas) - A web-based key-value viewer for kubernetes etcd3.0+ cluster.
+- [etcd-manager](https://github.com/gtamas/etcdmanager) - A modern, efficient, multi-platform and free ETCD 3.x GUI & client tool. Available for Windows, Linux and Mac.
 
 **Go libraries**