Add a "Join Slack" badge (#167)

Refs #157 (comment)

Author: jmini

README.md

@@ -4,6 +4,8 @@ Compare two OpenAPI specifications (3.x) and render the difference to HTML plain
 
 [![Test](https://github.com/OpenAPITools/openapi-diff/workflows/Test/badge.svg)](https://github.com/OpenAPITools/openapi-diff/actions?query=branch%3Amaster+workflow%3ATest+)
 
+[![Join the Slack chat room](https://img.shields.io/badge/Slack-Join%20the%20chat%20room-orange)](https://join.slack.com/t/openapi-generator/shared_invite/enQtNzAyNDMyOTU0OTE1LTY5ZDBiNDI5NzI5ZjQ1Y2E5OWVjMjZkYzY1ZGM2MWQ4YWFjMzcyNDY5MGI4NjQxNDBiMTlmZTc5NjY2ZTQ5MGM)
+
 # Requirements
 
 * Java 8