docs (README): add Gitter badge

README.md

@@ -7,6 +7,7 @@
 [![Awesome](https://awesome.re/badge-flat2.svg)](https://github.com/veelenga/awesome-crystal)
 [![vladfaust.com](https://img.shields.io/badge/style-.com-lightgrey.svg?longCache=true&style=flat-square&label=vladfaust&colorB=0a83d8)](https://vladfaust.com)
 [![Patrons count](https://img.shields.io/badge/dynamic/json.svg?label=patrons&url=https://www.patreon.com/api/user/11296360&query=$.included[0].attributes.patron_count&style=flat-square&colorB=red&maxAge=86400)](https://www.patreon.com/vladfaust)
+[![Gitter chat](https://img.shields.io/badge/chat%20on-gitter-green.svg?colorB=ED1965&logo=gitter&style=flat-square)](https://gitter.im/vladfaust/migrate.cr)
 
 A database migration tool for [Crystal](https://crystal-lang.org/).