chore: add coverage badge to README

Author: steve-chavez

README.md

@@ -1,5 +1,7 @@
 # plmustache
 
+[![Coverage Status](https://coveralls.io/repos/github/PostgREST/plmustache/badge.svg?branch=cov)](https://coveralls.io/github/PostgREST/plmustache?branch=cov)
+
 A PostgreSQL extension that provides a language handler for [Mustache](https://mustache.github.io/) templates using https://gitlab.com/jobol/mustach.
 
 ## Roadmap