Add code coverage badge

Author: atifaziz

index.html

@@ -30,6 +30,7 @@ <h2 class="project-tagline">Extensions to LINQ to Objects</h2>
     <p>
       <a href="https://gitter.im/morelinq/MoreLINQ"><img alt="Chat on Gitter" src="https://badges.gitter.im/morelinq/MoreLINQ.svg"></a>
       <a href="https://ci.appveyor.com/project/raboof/morelinq"><img alt="CI build status" src="https://img.shields.io/appveyor/ci/raboof/MoreLINQ/master.svg"></a>
+      <a href="https://codecov.io/gh/morelinq/MoreLINQ"><img src="https://codecov.io/gh/morelinq/MoreLINQ/graph/badge.svg?token=13hYYiGNe2"/></a>      
       <a href="https://www.nuget.org/packages/morelinq"><img alt="NuGet package (stable)" src="https://buildstats.info/nuget/morelinq"></a>
       <a href="https://www.myget.org/feed/morelinq/package/nuget/morelinq"><img alt="MyGet package (unstable)" src="https://img.shields.io/myget/morelinq/v/morelinq.svg?label=myget"></a>
     </p>