message: Add slack link to readme
  Summary: add slack link
diff: @@ -113,6 +113,9 @@ For more options, see
  We'd love to have your help in making `mvfst` better. If you're interested, please
  read our guide to [guide to contributing](CONTRIBUTING.md)
  +Please also join our
  +[slack](https://join.slack.com/t/mvfst/shared_invite/enQtNjE0ODIwNDU3MDU4LWFkOTc0ZTQ4NzczZmE2MjRlMjQxNWQxNDAyYzAzMDQ5MTQ0ZGI0YTJhZWM0MThhM2FiYzc0Zjg3MTYyNDg2MWI) to ask questions or start discussions.
  +
  ## License
  `mvfst` is MIT licensed, as found in the LICENSE file.
  