Add Coverage badge to README.md

Author: staticfloat

README.md

@@ -1,6 +1,7 @@
 # NNlib
 
-[![Build Status](https://travis-ci.org/FluxML/NNlib.jl.svg?branch=master)](https://travis-ci.org/FluxML/NNlib.jl) [![Build status](https://ci.appveyor.com/api/projects/status/wo2wkv1l9cj548uh?svg=true)](https://ci.appveyor.com/project/one-more-minute/nnlib-jl)
+[![Build Status](https://travis-ci.org/FluxML/NNlib.jl.svg?branch=master)](https://travis-ci.org/FluxML/NNlib.jl) [![Build status](https://ci.appveyor.com/api/projects/status/wo2wkv1l9cj548uh?svg=true)](https://ci.appveyor.com/project/one-more-minute/nnlib-jl) [![Coverage](https://codecov.io/gh/FluxML/NNlib/branch/master/graph/badge.svg)](https://codecov.io/gh/FluxML/NNlib)
+
 
 This package will provide a library of functions useful for ML, such as softmax, sigmoid, convolutions and pooling. It doesn't provide any other "high-level" functionality like layers or AD.