add coverage to readme

Author: TomWagg

README.md

@@ -11,6 +11,9 @@
   <a href='https://cogsworth.readthedocs.io/en/latest/?badge=latest'>
       <img src='https://readthedocs.org/projects/cogsworth/badge/?version=latest' alt='Documentation Status' />
   </a>
+  <a href="https://codecov.io/github/TomWagg/cogsworth" > 
+      <img src="https://codecov.io/github/TomWagg/cogsworth/graph/badge.svg?token=L2WGWNLJQU"/> 
+  </a>
   <a href="mailto:tomjwagg@gmail.com">
       <img src="https://img.shields.io/badge/contact-authors-blueviolet.svg?style=flat" alt="Email the authors"/>
   </a>