Add Ruff badge

README.md

@@ -2,6 +2,7 @@ Backend for Historical Manuscripts Indexing
 ===========================================
 
 [![pipeline status](https://gitlab.teklia.com/arkindex/backend/badges/master/pipeline.svg)](https://gitlab.teklia.com/arkindex/backend/commits/master)
+[![Ruff](https://img.shields.io/endpoint?url=https://raw.githubusercontent.com/astral-sh/ruff/main/assets/badge/v2.json)](https://github.com/astral-sh/ruff)
 
 ## Requirements