README: add Discourse link

Closes Homebrew#43524.

Signed-off-by: Sean Molenaar <[email protected]>

Author: EricFromCanada

README.md

@@ -4,6 +4,8 @@ Core formulae for the Homebrew package manager.
 
 Core formulae for the Linuxbrew package manager (Homebrew on Linux or Windows 10 Subsystem for Linux) are in [Homebrew/linuxbrew-core](https://github.com/Homebrew/linuxbrew-core).
 
+[![Join us on https://discourse.brew.sh](https://img.shields.io/badge/Discourse-forum-blue.svg)](https://discourse.brew.sh)
+
 ## How do I install these formulae?
 
 Just `brew install <formula>`. This is the default tap for Homebrew and is installed by default.