add Appveyor badge

Author: simonbyrne

README.md

@@ -1,6 +1,7 @@
 # Libm.jl
 
-[![Build Status](https://travis-ci.org/JuliaMath/Libm.jl.svg?branch=master)](https://travis-ci.org/JuliaMath/Libm.jl)
+[![Travis Build Status](https://travis-ci.org/JuliaMath/Libm.jl.svg?branch=master)](https://travis-ci.org/JuliaMath/Libm.jl)
+[![Appveyor Build Status](https://ci.appveyor.com/api/projects/status/307l6b799amrpvks/branch/master?svg=true)](https://ci.appveyor.com/project/simonbyrne/libm-jl/branch/master)
 [![Coverage Status](https://coveralls.io/repos/JuliaMath/Libm.jl/badge.svg?branch=master&service=github)](https://coveralls.io/github/JuliaMath/Libm.jl?branch=master)
 [![codecov.io](http://codecov.io/github/JuliaMath/Libm.jl/coverage.svg?branch=master)](http://codecov.io/github/JuliaMath/Libm.jl?branch=master)