doc: add mixer/etcd3 as a Node.js client integration
**Node libraries**
+- [mixer/etcd3](https://github.com/mixer/etcd3) - Supports v3
- [stianeikeland/node-etcd](https://github.com/stianeikeland/node-etcd) - Supports v2 (w Coffeescript)
- [lavagetto/nodejs-etcd](https://github.com/lavagetto/nodejs-etcd) - Supports v2
- [deedubs/node-etcd-config](https://github.com/deedubs/node-etcd-config) - Supports v2
