added stable docs badge

Author: zsunberg

README.md

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 | [![Build Status](https://travis-ci.org/JuliaPOMDP/POMDPs.jl.svg?branch=master)](https://travis-ci.org/JuliaPOMDP/POMDPs.jl) | [![Build Status](https://travis-ci.org/JuliaPOMDP/POMDPs.jl.svg?branch=master)](https://travis-ci.org/JuliaPOMDP/POMDPs.jl)| [![Build Status](https://travis-ci.org/JuliaPOMDP/POMDPs.jl.svg?branch=master)](https://travis-ci.org/JuliaPOMDP/POMDPs.jl)|
 
-[![Docs](https://img.shields.io/badge/docs-latest-blue.svg)](https://JuliaPOMDP.github.io/POMDPs.jl/latest)
+[![Docs](https://img.shields.io/badge/docs-stable-blue.svg)](https://JuliaPOMDP.github.io/POMDPs.jl/stable)
+[![Dev-Docs](https://img.shields.io/badge/docs-latest-blue.svg)](https://JuliaPOMDP.github.io/POMDPs.jl/latest)
 [![Gitter](https://badges.gitter.im/JuliaPOMDP/Lobby.svg)](https://gitter.im/JuliaPOMDP/Lobby?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge)
 
 This package provides a core interface for working with [Markov decision processes (MDPs)](https://en.wikipedia.org/wiki/Markov_decision_process) and [partially observable Markov decision processes (POMDPs)](https://en.wikipedia.org/wiki/Partially_observable_Markov_decision_process). For examples, please see [POMDPExamples](https://github.com/JuliaPOMDP/POMDPExamples.jl), [QuickPOMDPs](https://github.com/JuliaPOMDP/QuickPOMDPs.jl), and the [Gallery](https://github.com/JuliaPOMDP/POMDPGallery.jl).