add a pre-commit badge to README (#2534)

README.md

@@ -9,6 +9,7 @@
 [![Docker Build Status](https://img.shields.io/docker/cloud/build/esmvalgroup/esmvalcore)](https://hub.docker.com/r/esmvalgroup/esmvalcore/)
 [![Anaconda-Server Badge](https://img.shields.io/conda/vn/conda-forge/ESMValCore?color=blue&label=conda-forge&logo=conda-forge&logoColor=white)](https://anaconda.org/conda-forge/esmvalcore)
 [![Github Actions Test](https://github.com/ESMValGroup/ESMValCore/actions/workflows/run-tests.yml/badge.svg)](https://github.com/ESMValGroup/ESMValCore/actions/workflows/run-tests.yml)
+[![pre-commit.ci status](https://results.pre-commit.ci/badge/github/ESMValGroup/ESMValCore/main.svg)](https://results.pre-commit.ci/latest/github/ESMValGroup/ESMValCore/main)
 
 ![esmvaltoollogo](https://raw.githubusercontent.com/ESMValGroup/ESMValCore/main/doc/figures/ESMValTool-logo-2-glow.png)