Update README.md

Author: MikeInnes

README.md

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 # MacroTools.jl
 
 [![Build Status](https://travis-ci.org/MikeInnes/MacroTools.jl.svg?branch=master)](https://travis-ci.org/MikeInnes/MacroTools.jl)
+[![](https://img.shields.io/badge/docs-stable-blue.svg)](https://mikeinnes.github.io/MacroTools.jl/stable)
 
 MacroTools provides a library of tools for working with Julia code and expressions. This includes a powerful template-matching system and code-walking tools that let you do deep transformations of code in a few lines. See the [docs](http://mikeinnes.github.io/MacroTools.jl/) for more info.