docs: add contributing section (#213)

Adds contributing section to indicate how to add apps to app showcase

Author: alexdanilowicz

README.md

@@ -16,3 +16,11 @@ npm start
 ## Project Stucture and Contributing
 
 For details, see [the docusarus docs](https://v2.docusaurus.io)
+
+## Contributing
+
+### Adding to App Showcase
+
+If you built an app with ERB and would like to add your app to the showcase, please add it to the end of the list.
+
+Please be sure to also include a screenshot or a .gif.