docs: link the Matrix room from the README

Author: stefan-ernst

README.md

@@ -9,6 +9,7 @@
   <a href="https://windshift.sh/download"><img src="https://img.shields.io/badge/download-latest-2e7dbd?style=flat-square" alt="Download"></a>
   <a href="LICENSE"><img src="https://img.shields.io/badge/license-AGPL--3.0-2e7dbd?style=flat-square" alt="AGPL-3.0 License"></a>
   <a href="https://windshift.sh/docs"><img src="https://img.shields.io/badge/docs-windshift.sh-2e7dbd?style=flat-square" alt="Documentation"></a>
+  <a href="https://matrix.to/#/%23windshift:matrix.org"><img src="https://img.shields.io/badge/matrix-%23windshift-2e7dbd?style=flat-square" alt="Matrix chat"></a>
 </p>
 
 <p align="center"> <strong>Work management that fits the way your team works.</strong><br> Plan projects, organize your work, and keep things moving, while keeping your data under your control. </p>