Update README.md - add gitter chat badge

Author: kobylynskyi

README.md

@@ -2,6 +2,7 @@
 
 ![Build](https://github.com/kobylynskyi/graphql-java-codegen/workflows/Build/badge.svg)
 [![Codecov](https://img.shields.io/codecov/c/github/kobylynskyi/graphql-java-codegen)](https://codecov.io/gh/kobylynskyi/graphql-java-codegen)
+[![Gitter](https://badges.gitter.im/graphql-java-codegen/community.svg)](https://gitter.im/graphql-java-codegen/community?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge)
 [![License: MIT](https://img.shields.io/badge/License-MIT-yellow.svg)](https://opensource.org/licenses/MIT)
 
 ## Plugins