Add Slack channel badge to the README (#448)

Summary:
This PR only adds a badge with the link to the ktmft Slack channel in Kotlinlang.

> [!Warning]
> The Facebook build badge is broken, if not, they would appear inline.

Pull Request resolved: #448

Reviewed By: hick209

Differential Revision: D55482265

Pulled By: cortinico

fbshipit-source-id: 16ccf68a6504fc221f221f7c8425ba4e30258b69

README.md

@@ -1,4 +1,4 @@
-[![](https://github.com/facebookincubator/ktfmt/workflows/build/badge.svg)](https://github.com/facebookincubator/ktfmt/actions?query=workflow%3Abuild)
+[![](https://github.com/facebookincubator/ktfmt/workflows/build/badge.svg)](https://github.com/facebookincubator/ktfmt/actions?query=workflow%3Abuild) [![slack](https://img.shields.io/badge/Slack-ktfmt-purple.svg?logo=slack)](https://slack-chats.kotlinlang.org/c/ktfmt) [![invite](https://img.shields.io/badge/Request%20a%20Slack%20invite-8A2BE2)](https://surveys.jetbrains.com/s3/kotlin-slack-sign-up)
 
 # ktfmt