Add codecov.io badge for master branch
Add codecov.io badge for master branch

kylekatarnls/Carbon,nicholas-eden/Carbon,Klinche/Carbon,m4nt4s/Carbon,damnedest/Carbon,dlepaux/Carbon,briannesbitt/Carbon,EazyServer/Carbon,hinternet/Carbon,serima/Carbon,lucasmichot/Carbon,lndj/Carbon