Add CI status badge

Author: Pierre-siddall

README.md

@@ -1,5 +1,7 @@
 # GRoWSS - GitHub Reusable Workflows for Simulation Systems :recycle:
 
+[![CI](https://github.com/MetOffice/growss/actions/workflows/validate.yaml/badge.svg?branch=main)](https://github.com/MetOffice/growss/actions/workflows/validate.yaml)
+
 Placeholder for a collection of
 [reusable workflows](https://docs.github.com/en/actions/learn-github-actions/reusing-workflows)
 for the Met Office Simulation Systems Repositories.