Mention our dev room in the contributing guide (#30714)

* Mention our dev room in the contributing guide

It was in there, but only in the tests section, relating to how
to write tests. This adds it in the first section.

* Prettier

Author: dbkr

CONTRIBUTING.md

@@ -2,6 +2,11 @@
 
 Everyone is welcome to contribute code to Element Web, provided that they are willing to license their contributions to Element under a [Contributor License Agreement](https://cla-assistant.io/element-hq/element-web) (CLA). This ensures that their contribution will be made available under an OSI-approved open-source license, currently licensed under Affero General Public License v3 (AGPLv3) or General Public License v3 (GPLv3) at your choice.
 
+If you're contributing, or thinking about contributing, please come & chat to
+us in our development room, [#element-dev](https://matrix.to/#/#element-dev:matrix.org).
+This is the best place to ask questions about the code, how to work on the project
+or whether a change is likely to be accepted.
+
 ## How to contribute
 
 The preferred and easiest way to contribute changes to the project is to fork