Add a matrix chat badge to the README (#9476)

* Add a matrix chat badge to the README

This is a fun addition for connecting users to our dedicated Substrate chat & to visualize the number of members there.
```
[![Matrix](https://img.shields.io/matrix/substrate-technical:matrix.org)](https://matrix.to/#/#substrate-technical:matrix.org)
```

https://img.shields.io/matrix/substrate-technical:matrix.org can be viewed directly to see the count before merging the PR here.

* Update README.md

substrate/README.md

-# Substrate · [![GitHub license](https://img.shields.io/badge/license-GPL3%2FApache2-blue)](#LICENSE) [![GitLab Status](https://gitlab.parity.io/parity/substrate/badges/master/pipeline.svg)](https://gitlab.parity.io/parity/substrate/pipelines) [![PRs Welcome](https://img.shields.io/badge/PRs-welcome-brightgreen.svg)](docs/CONTRIBUTING.adoc)
+# Substrate · [![GitHub license](https://img.shields.io/badge/license-GPL3%2FApache2-blue)](#LICENSE) [![GitLab Status](https://gitlab.parity.io/parity/substrate/badges/master/pipeline.svg)](https://gitlab.parity.io/parity/substrate/pipelines) [![PRs Welcome](https://img.shields.io/badge/PRs-welcome-brightgreen.svg)](docs/CONTRIBUTING.adoc) [![Matrix](https://img.shields.io/matrix/substrate-technical:matrix.org)](https://matrix.to/#/#substrate-technical:matrix.org)
 
 <p align="center">
   <img src="/docs/media/sub.gif">