add license + badge

README.md

 # CI templates for R2DevOps open source catalog
 
-[![Latest Release](https://lab.frogg.it/dorianturba/r2devops_catalog/-/badges/release.svg?order_by=release_at)](https://lab.frogg.it/r2devops_catalog/fake_session_maker/-/releases)
+[![License: MIT](https://img.shields.io/badge/License-MIT-yellow.svg)](https://lab.frogg.it/dorianturba/r2devops_catalog/-/blob/main/LICENSE)
 [![Pipeline](https://lab.frogg.it/dorianturba/r2devops_catalog/badges/main/pipeline.svg)](https://lab.frogg.it/dorianturba/r2devops_catalog/-/pipelines)
 [![Markdown: pymarkdown](https://img.shields.io/badge/%20markdown-pymarkdown-%231674b1?style=flat&labelColor=ef8336)](https://github.com/jackdewinter/pymarkdown)
+[![Mattermost-R2DevopsCatalog](https://img.shields.io/badge/Mattermost-R2DevopsCatalog-%2379eb00?style=flat&logo=mattermost)](https://chat.froggit.fr/signup_user_complete/?id=e6r8p645fbbut8wubbdph9ot9w&sbr=su)
 
 This repository contains CI templates dedicated to Python software testing for the
 R2DevOps open source catalog.