MNT: Update README chat badge

We don't use gitter any more.

README.md

@@ -2,7 +2,7 @@
 
 ![Code style: black](https://img.shields.io/badge/code%20style-black-000000.svg)
 [![Coverage](https://codecov.io/gh/dials/dials/branch/main/graph/badge.svg)](https://codecov.io/gh/dials/dials)
-[![Gitter](https://badges.gitter.im/dials/community.svg)](https://gitter.im/dials/community?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge)
+[![Slack](https://img.shields.io/badge/chat-Slack-green)](https://join.slack.com/t/dials-support/shared_invite/zt-21fvg5n53-SG~882mWRs189GSMuemPfg)
 
 X-ray crystallography for structural biology has benefited greatly from a number of advances in recent years including high performance pixel array detectors, new beamlines capable of delivering micron and sub-micron focus and new light sources such as XFELs. The DIALS project is a collaborative endeavour to develop new diffraction integration software to meet the data analysis requirements presented by these recent advances. There are three end goals: to develop an extensible framework for the development of algorithms to analyse X-ray diffraction data; the implementation of algorithms within this framework and finally a set of user facing tools using these algorithms to allow integration of data from diffraction experiments on synchrotron and free electron sources.