add GH discussions link to README (#866)

* add GH discussions link to README

* add link to MMM hub slack

* Update README.md

---------

Co-authored-by: Will Dean <[email protected]>

Author: cluhmann

README.md

@@ -34,8 +34,10 @@ Explore these topics further by watching our video on [Bayesian Marketing Mix Mo
 
 ### Community Resources
 
-- [Bayesian discord server](https://discord.gg/swztKRaVKe)
+- [PyMC-Marketing Discussions](https://github.com/pymc-labs/pymc-marketing/discussions)
 - [PyMC discourse](https://discourse.pymc.io/)
+- [Bayesian discord server](https://discord.gg/swztKRaVKe)
+- [MMM Hub Slack](https://www.mmmhub.org/slack)
 
 ## Quick Installation Guide for Marketing Mix Modeling (MMM) & CLV