docs: add mypy badge

README.md

@@ -2,6 +2,7 @@
 
 ![tests](https://github.com/acdh-oeaw/rdfproxy/actions/workflows/tests.yaml/badge.svg)
 [![coverage](https://coveralls.io/repos/github/acdh-oeaw/rdfproxy/badge.svg?branch=main&kill_cache=1)](https://coveralls.io/github/acdh-oeaw/rdfproxy?branch=main&kill_cache=1)
+![mypy](https://github.com/acdh-oeaw/rdfproxy/actions/workflows/mypy.yml/badge.svg)
 [![docs](https://github.com/acdh-oeaw/rdfproxy/actions/workflows/deploy-docs.yml/badge.svg)](https://acdh-oeaw.github.io/rdfproxy/)
 [![License: GPL v3](https://img.shields.io/badge/License-GPLv3-blue.svg)](https://www.gnu.org/licenses/gpl-3.0)
 [![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)