Add a code coverage badge to the README file

Author: stevegrunwell

README.md

@@ -1,7 +1,8 @@
 # PHPUnit Markup Assertions
 
 ![Build Status](https://github.com/stevegrunwell/phpunit-markup-assertions/workflows/Unit%20Tests/badge.svg)
-[![GitHub release](https://img.shields.io/github/release/stevegrunwell/phpunit-markup-assertions.svg)](https://github.com/stevegrunwell/phpunit-markup-assertions/releases)
+[![Code Coverage](https://coveralls.io/repos/github/stevegrunwell/phpunit-markup-assertions/badge.svg?branch=develop)](https://coveralls.io/github/stevegrunwell/phpunit-markup-assertions?branch=develop)
+[![GitHub Release](https://img.shields.io/github/release/stevegrunwell/phpunit-markup-assertions.svg)](https://github.com/stevegrunwell/phpunit-markup-assertions/releases)
 
 This library introduces the `MarkupAssertionsTrait` trait for use in [PHPUnit](https://phpunit.de) tests.