[README.md] add coverage badge

Author: chrissimpkins

README.md

@@ -2,4 +2,6 @@
 
 [![Build Status](https://travis-ci.org/source-foundry/fdiff.svg?branch=master)](https://travis-ci.org/source-foundry/fdiff)
 [![Build status](https://ci.appveyor.com/api/projects/status/eafvbkc4iyv78dip/branch/master?svg=true)](https://ci.appveyor.com/project/chrissimpkins/fdiff/branch/master)
+[![codecov](https://codecov.io/gh/source-foundry/fdiff/branch/master/graph/badge.svg)](https://codecov.io/gh/source-foundry/fdiff)
+