Add coverage badge to README

Shows 100% test coverage status

Author: raghubetina

README.md

@@ -3,6 +3,7 @@
 [![CI](https://github.com/firstdraft/normalized_match/actions/workflows/ci.yml/badge.svg)](https://github.com/firstdraft/normalized_match/actions/workflows/ci.yml)
 [![Gem Version](https://badge.fury.io/rb/normalized_match.svg)](https://badge.fury.io/rb/normalized_match)
 [![Ruby Style Guide](https://img.shields.io/badge/code_style-standard-brightgreen.svg)](https://github.com/standardrb/standard)
+[![Coverage Status](https://img.shields.io/badge/coverage-100%25-brightgreen.svg)](https://github.com/firstdraft/normalized_match)
 
 A normalized string matcher for RSpec that ignores case, punctuation, and some whitespace differences.