Add matrix chat badge (#106)

Author: general-CbIC

README.md

@@ -5,6 +5,7 @@
 [![Hex Docs](https://img.shields.io/badge/hex-docs-lightgreen.svg?style=flat)](https://hexdocs.pm/poolex/)
 [![License](https://img.shields.io/hexpm/l/poolex.svg?style=flat)](https://github.com/general-CbIC/poolex/blob/main/LICENSE)
 [![Total Download](https://img.shields.io/hexpm/dt/poolex.svg?style=flat)](https://hex.pm/packages/poolex)
+[![Chat on Matrix](https://matrix.to/img/matrix-badge.svg?style=flat)](https://matrix.to/#/#poolex:gitter.im)
 
 Poolex is a library for managing pools of workers. Inspired by [poolboy](https://github.com/devinus/poolboy).