add lgtm badge

Author: kay0u

README.md

@@ -4,6 +4,7 @@
 
 ![Version](https://img.shields.io/github/v/tag/yunohost/moulinette?label=version&sort=semver)
 [![Tests status](https://github.com/YunoHost/moulinette/actions/workflows/tox.yml/badge.svg)](https://github.com/YunoHost/moulinette/actions/workflows/tox.yml)
+[![Language grade: Python](https://img.shields.io/lgtm/grade/python/g/YunoHost/moulinette.svg?logo=lgtm&logoWidth=18)](https://lgtm.com/projects/g/YunoHost/moulinette/context:python)
 [![GitHub license](https://img.shields.io/github/license/YunoHost/moulinette)](https://github.com/YunoHost/moulinette/blob/dev/LICENSE)