add macos testing badge

Author: robertjwilson

README.md

@@ -1,6 +1,7 @@
 [![Documentation Status](https://readthedocs.org/projects/oceanval/badge/?version=latest)](https://oceanval.readthedocs.io/en/latest/?badge=latest)
 [![Conda Badge](https://anaconda.org/conda-forge/oceanval/badges/version.svg)](https://anaconda.org/channels/conda-forge/packages/oceanval/overview)
 ![GitHub Testing](https://github.com/pmlmodelling/oceanVal/actions/workflows/python-app.yml/badge.svg)
+![GitHub Testing](https://github.com/pmlmodelling/oceanVal/actions/workflows/python-app-macos.yml/badge.svg)
 [![codecov](https://codecov.io/gh/pmlmodelling/oceanVal/graph/badge.svg?token=RLHUN0F9W7)](https://codecov.io/gh/pmlmodelling/oceanVal)