README: Add codecov.io badge

Shows the current code coverage %.

Author: angularsen

README.md

@@ -1,4 +1,5 @@
-[![Build status](https://ci.appveyor.com/api/projects/status/f8qfnqd7enkc6o4k/branch/master?svg=true)](https://ci.appveyor.com/project/angularsen/unitsnet/history/branch/master) [![Join the chat at https://gitter.im/UnitsNet/Lobby](https://badges.gitter.im/UnitsNet/Lobby.svg)](https://gitter.im/UnitsNet/Lobby?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge) 
+[![Build status](https://ci.appveyor.com/api/projects/status/f8qfnqd7enkc6o4k/branch/master?svg=true)](https://ci.appveyor.com/project/angularsen/unitsnet/history/branch/master) [![Join the chat at https://gitter.im/UnitsNet/Lobby](https://badges.gitter.im/UnitsNet/Lobby.svg)](https://gitter.im/UnitsNet/Lobby?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
+[![codecov](https://codecov.io/gh/angularsen/UnitsNet/branch/master/graph/badge.svg)](https://codecov.io/gh/angularsen/UnitsNet)
 
 ## Units.NET