Add Gitter badge

Author: gitter-badger

README.md

@@ -3,7 +3,7 @@
 [![Go](https://github.com/huandu/go-sqlbuilder/workflows/Go/badge.svg)](https://github.com/huandu/go-sqlbuilder/actions)
 [![GoDoc](https://godoc.org/github.com/huandu/go-sqlbuilder?status.svg)](https://pkg.go.dev/github.com/huandu/go-sqlbuilder)
 [![Go Report](https://goreportcard.com/badge/github.com/huandu/go-sqlbuilder)](https://goreportcard.com/report/github.com/huandu/go-sqlbuilder)
-[![Coverage Status](https://coveralls.io/repos/github/huandu/go-sqlbuilder/badge.svg?branch=master)](https://coveralls.io/github/huandu/go-sqlbuilder?branch=master)
+[![Coverage Status](https://coveralls.io/repos/github/huandu/go-sqlbuilder/badge.svg?branch=master)](https://coveralls.io/github/huandu/go-sqlbuilder?branch=master) [![Join the chat at https://gitter.im/go-sqlbuilder/community](https://badges.gitter.im/go-sqlbuilder/community.svg)](https://gitter.im/go-sqlbuilder/community?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 
 - [Install](#install)
 - [Usage](#usage)