Add EffVer badge

Author: jacobtomlinson

README.md

@@ -12,6 +12,7 @@
 [![Azure pipelines status](https://dev.azure.com/matplotlib/matplotlib/_apis/build/status/matplotlib.matplotlib?branchName=main)](https://dev.azure.com/matplotlib/matplotlib/_build/latest?definitionId=1&branchName=main)
 [![AppVeyor status](https://ci.appveyor.com/api/projects/status/github/matplotlib/matplotlib?branch=main&svg=true)](https://ci.appveyor.com/project/matplotlib/matplotlib)
 [![Codecov status](https://codecov.io/github/matplotlib/matplotlib/badge.svg?branch=main&service=github)](https://app.codecov.io/gh/matplotlib/matplotlib)
+[![EffVer Versioning](https://img.shields.io/badge/version_scheme-EffVer-0097a7)](https://jacobtomlinson.dev/effver)
 
 ![Matplotlib logotype](https://matplotlib.org/_static/logo2.svg)