Document: add lucas to integrations.md
add lucas to integrations.md
- [etcd-rest](https://github.com/mickep76/etcd-rest) - Create generic REST API in Go using etcd as a backend with validation using JSON schema
- [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.
**Go libraries**
