add link to CSS-focused Matrix chatroom to README.md

[TallTed]

📁 Related issues

✍️ Description

✅ PR check list

Before this pull request can be merged, a core maintainer will check whether

• this PR is labeled with the correct semver label
  • semver.patch: Backwards compatible bug fixes.
  • semver.minor: Backwards compatible feature additions.
  • semver.major: Breaking changes. This includes changing interfaces or configuration behaviour.
• the correct branch is targeted. Patch updates can target main, other changes should target the latest versions/* branch.
• the RELEASE_NOTES.md document in case of relevant feature or config changes.
• any relevant documentation was updated to reflect the changes in this PR.

[joachimvh]

Not a bad idea