Add coverage badge to README

As suggested in JuliaLang#9493.

Author: kshyatt

README.md

@@ -9,6 +9,8 @@ Linux, OSX: [![Build Status](https://api.travis-ci.org/JuliaLang/julia.svg?branc
 
 Windows: [![Build status](https://ci.appveyor.com/api/projects/status/4vr88cmgo7u02644/branch/master?svg=true)](https://ci.appveyor.com/project/StefanKarpinski/julia/branch/master)
 
+Code Coverage: [![Coverage Status](https://coveralls.io/repos/JuliaLang/julia/badge.svg?branch=master)](https://coveralls.io/r/JuliaLang/julia?branch=master)
+
 <a name="The-Julia-Language"/>
 ## The Julia Language