Update README.md

Author: SimonDanisch

README.md

@@ -2,6 +2,7 @@
 
 [![Build Status](https://travis-ci.org/JuliaGPU/GPUArrays.jl.svg?branch=master)](https://travis-ci.org/JuliaGPU/GPUArrays.jl)
 [![Build status](https://ci.appveyor.com/api/projects/status/2aa4bvmq7e9rh338/branch/master?svg=true)](https://ci.appveyor.com/project/SimonDanisch/gpuarrays-jl-8n74h/branch/master)
+[![](https://img.shields.io/badge/docs-latest-blue.svg)](https://JuliaGPU.github.io/GPUArrays.jl/latest)
 
 [Benchmarks](https://github.com/JuliaGPU/GPUBenchmarks.jl/blob/master/results/results.md)