Add code coverage badge to README.md

Author: sharwell

README.md

@@ -1,6 +1,7 @@
 # Mouse Fast Scroll extension for Visual Studio 2012+
 
 [![Build status](https://ci.appveyor.com/api/projects/status/bxcpp55od7oo11si/branch/master?svg=true)](https://ci.appveyor.com/project/sharwell/mousefastscroll/branch/master)
+[![codecov.io](http://codecov.io/gh/tunnelvisionlabs/MouseFastScroll/coverage.svg?branch=master)](http://codecov.io/gh/tunnelvisionlabs/MouseFastScroll?branch=master)
 
 This extension performs two tasks that I simply can't live without: