doc: badge

README.md

@@ -1,6 +1,7 @@
 # COATJAVA
 [![Build Status](https://github.com/jeffersonlab/coatjava/workflows/Coatjava-CI/badge.svg)](https://github.com/jeffersonlab/coatjava/actions)
 [![Validation Status](https://github.com/JeffersonLab/coatjava/actions/workflows/validation.yml/badge.svg)](https://github.com/JeffersonLab/coatjava/actions/workflows/validation.yml)
+[![Coverage](https://badgen.net/static/JaCoCo/coverage/purple)](https://jeffersonlab.github.io/coatjava)
 
 The original repository for COATJAVA was named "clas12-offline-software" and is [now archived and read-only](https://github.com/JeffersonLab/clas12-offline-software).  On May 17, 2023, this new repository was created by running BFG Repo Cleaner to get rid of old, large data files and things that should never have been in the repository, giving 10x reduction in repository size, clone time, etc, and renamed `coatjava`.  The most critical, GitHub-specific aspects have been transferred to this new repository: