Update documentation with forum (#987)

Author: joeldrapper

README.md

@@ -1,10 +1,16 @@
 # Phlex
 
+[![Ruby Users Forum](https://img.shields.io/discourse/topics?server=https%3A%2F%2Fwww.rubyforum.org&style=flat&logo=discourse&label=Ruby%20Users%20Forum)](https://www.rubyforum.org/tag/phlex)
+
 Phlex lets you build object-oriented web views in pure Ruby.
 
 - [v2 Docs](https://www.phlex.fun)
 - [v1 Docs](https://v1.phlex.fun)
 
+## Community
+
+Join us in the `phlex` tag on the [Ruby Users Forum](https://www.rubyforum.org/tag/phlex).
+
 ## Versioning and Maintenance
 
 Phlex does not follow Semantic Versioning (SemVer). Instead, we follow [BreakVer](https://www.taoensso.com/break-versioning).

phlex.gemspec

@@ -19,6 +19,7 @@ Gem::Specification.new do |spec|
 	spec.metadata["changelog_uri"] = "https://github.com/phlex-ruby/phlex/releases"
 	spec.metadata["funding_uri"] = "https://github.com/sponsors/joeldrapper"
 	spec.metadata["rubygems_mfa_required"] = "true"
+	spec.metadata["mailing_list_uri"] = "https://www.rubyforum.org/tag/phlex"
 
 	spec.files = Dir[
 		"README.md",