clientv3: add link to architecture docs
# etcd/clientv3
+[![Docs](https://readthedocs.org/projects/etcd/badge/?version=latest&style=flat-square)](https://etcd.readthedocs.io/en/latest/?badge=latest)
[![Godoc](https://img.shields.io/badge/go-documentation-blue.svg?style=flat-square)](https://godoc.org/github.com/coreos/etcd/clientv3)
`etcd/clientv3` is the official Go etcd client for v3.
+See https://etcd.readthedocs.io/en/latest for latest client architecture.
+
## Install
```bash
