feat: added slack badges to README

Author: libremente

README.md

@@ -2,6 +2,8 @@
 
 ![Repo Status:
 Alpha](https://img.shields.io/badge/status-alpha-lightgrey.svg?longCache=true&style=plastic)
+[![Join the #publiccode channel](https://img.shields.io/badge/Slack%20channel-%23design-blue.svg)](https://developersitalia.slack.com/messages/CAM3F785T/)
+[![Get invited](https://slack.developers.italia.it/badge.svg)](https://slack.developers.italia.it/)
 
 A metadata description standard for public software and policy repositories
 that is easy to use both for developers and people with less technical