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