Update README badge (#324)

[ci skip]

README.md

@@ -4,9 +4,10 @@
 [![Windows Build](https://ci.appveyor.com/api/projects/status/re90njols2th2ide?svg=true)](https://ci.appveyor.com/project/pluskid/mxnet-jl)
 [![codecov.io](https://codecov.io/github/dmlc/MXNet.jl/coverage.svg?branch=master)](https://codecov.io/github/dmlc/MXNet.jl?branch=master)
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-[![MXNet](http://pkg.julialang.org/badges/MXNet_0.4.svg)](http://pkg.julialang.org/?pkg=MXNet)
+[![](https://img.shields.io/badge/docs-stable-blue.svg)](https://dmlc.github.io/MXNet.jl/stable)
+[![MXNet](http://pkg.julialang.org/badges/MXNet_0.6.svg)](http://pkg.julialang.org/?pkg=MXNet)
 [![License](http://dmlc.github.io/img/apache2.svg)](LICENSE.md)
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 MXNet.jl is the [dmlc/mxnet](https://github.com/dmlc/mxnet) [Julia](http://julialang.org/) package. MXNet.jl brings flexible and efficient GPU computing and state-of-art deep learning to Julia. Some highlight of its features include: