add badge to README

README.md

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 [![Build status](https://img.shields.io/github/actions/workflow/status/digitalcredentials/transaction-service/main.yml?branch=main)](https://github.com/digitalcredentials/transaction-service/actions?query=workflow%3A%22Node.js+CI%22)
 
+[![Coverage Status](https://coveralls.io/repos/github/digitalcredentials/transaction-service/badge.svg?branch=jc-add-healthz)](https://coveralls.io/github/digitalcredentials/transaction-service?branch=jc-add-healthz)
+
 > Express app for managing the transactions used in [VC-API exchanges](https://w3c-ccg.github.io/vc-api/#initiate-exchange).
 
 #### IMPORTANT NOTE ABOUT VERSIONING: If you are using a Docker Hub image of this repository, make sure you are reading the version of this README that corresponds to your Docker Hub version.  If, for example, you are using the image `digitalcredentials/transaction-service:0.1.0` then you'll want to use the corresponding tagged repo: [https://github.com/digitalcredentials/transaction-service/tree/v0.1.0](https://github.com/digitalcredentials/transaction-service/tree/v0.1.0). If you are new here, then just read on...