[Docs]: Slack link in Readme

[f0x52]

The join us on Slack badge links to https://coder.co/join-community, it should probably be https://coder.com/community

[jsjoeio]

Oops! Thanks for catching!

[bpmct]

Ah, this must have happened when we bulk-renamed cdr to coder. https://cdr.co/join-community is, in fact, a valid URL.

https://coder.com/community is more comprehensive and offers more resources, though :)