Add a badge to the README with the ktfmt channel in Kotlinlang

[JavierSegoviaCordoba]

Like a lot of Kotlin utilities, there is a channel for ktfmt in the kotlinlang Slack, should be great to add it to the README:

https://kotlinlang.slack.com/archives/C01GZCU0QNB

[cgrushko]

Ah, very cool :) let me get an invite, I'd like to see what's in there.

[JavierSegoviaCordoba]

https://surveys.jetbrains.com/s3/kotlin-slack-sign-up

[hick209]

We have added the Koltin slack channel here, but I think it would be valuable to add that in the README as well and with the link for joining.

@JavierSegoviaCordoba, do you want to submit a quick PR adding this?

[JavierSegoviaCordoba]

Done: #448