Add social badges (#2142)

README.md

-# code-server
+# code-server · [!["GitHub Discussions"](https://img.shields.io/badge/%20GitHub-%20Discussions-gray.svg?longCache=true&logo=github&colorB=purple)](https://github.com/cdr/code-server/discussions) [!["Join us on Slack"](https://img.shields.io/badge/join-us%20on%20slack-gray.svg?longCache=true&logo=slack&colorB=brightgreen)](https://cdr.co/join-community) [![Twitter Follow](https://img.shields.io/twitter/follow/CoderHQ?label=%40CoderHQ&style=social)](https://twitter.com/coderhq)
 
 Run [VS Code](https://github.com/Microsoft/vscode) on any machine anywhere and access it in the browser.