Add Community Section. (#954)

To continue to drive awareness to our CloudFormation Discord and give the community another place to ask questions we are adding a community section to our popular repositories. 

I placed this at the bottom of the ReadMe but open to suggestions on where in the ReadMe this section should be placed.

Author: cmgorton

README.md

@@ -145,3 +145,11 @@ Plugins must provide the same interface as `LanguagePlugin` (in `plugin_base.py`
 ## License
 
 This library is licensed under the Apache 2.0 License.
+
+## Community
+
+Join us on Discord! Connect & interact with CloudFormation developers &
+experts, find channels to discuss and get help for our CLI, cfn-lint, CloudFormation registry, StackSets,
+Guard and more:
+
+[![Join our Discord](https://discordapp.com/api/guilds/981586120448020580/widget.png?style=banner3)](https://discord.gg/9zpd7TTRwq)