Add Google Group badge
<img src="doc/logo-text.png" height=100></img>
-[![Circle CI](https://circleci.com/gh/rqlite/rqlite/tree/master.svg?style=svg)](https://circleci.com/gh/rqlite/rqlite/tree/master) [![appveyor](https://ci.appveyor.com/api/projects/status/github/rqlite/rqlite?branch=master&svg=true)](https://ci.appveyor.com/project/otoolep/rqlite) [![GoDoc](https://godoc.org/github.com/rqlite/rqlite?status.svg)](https://godoc.org/github.com/rqlite/rqlite) [![Go Report Card](https://goreportcard.com/badge/github.com/rqlite/rqlite)](https://goreportcard.com/report/github.com/rqlite/rqlite) [![Release](https://img.shields.io/github/release/rqlite/rqlite.svg)](https://github.com/rqlite/rqlite/releases)
+[![Circle CI](https://circleci.com/gh/rqlite/rqlite/tree/master.svg?style=svg)](https://circleci.com/gh/rqlite/rqlite/tree/master) [![appveyor](https://ci.appveyor.com/api/projects/status/github/rqlite/rqlite?branch=master&svg=true)](https://ci.appveyor.com/project/otoolep/rqlite) [![GoDoc](https://godoc.org/github.com/rqlite/rqlite?status.svg)](https://godoc.org/github.com/rqlite/rqlite) [![Go Report Card](https://goreportcard.com/badge/github.com/rqlite/rqlite)](https://goreportcard.com/report/github.com/rqlite/rqlite) [![Release](https://img.shields.io/github/release/rqlite/rqlite.svg)](https://github.com/rqlite/rqlite/releases) [![Google Group](https://img.shields.io/badge/Google%20Group--blue.svg)](https://groups.google.com/group/rqlite)
*rqlite* is a distributed relational database, which uses [SQLite](https://www.sqlite.org/) as its storage engine. rqlite uses [Raft](http://raftconsensus.github.io/) to achieve consensus across all the instances of the SQLite databases, ensuring that every change made to the system is made to a quorum of SQLite databases, or none at all. It also gracefully handles leader elections, and tolerates failures of machines, including the leader. rqlite is available for Linux, OSX, and Microsoft Windows.
