Add gitter badge (#638)

README.md

@@ -7,6 +7,8 @@ An image processing library for [Julia](http://julialang.org/).
 [![Build Status](https://travis-ci.org/JuliaImages/Images.jl.svg?branch=master)](https://travis-ci.org/JuliaImages/Images.jl)
 [![Build status](https://ci.appveyor.com/api/projects/status/github/JuliaImages/Images.jl?svg=true&branch=master)](https://ci.appveyor.com/project/timholy/images-jl/branch/master)
 [![codecov](https://codecov.io/gh/JuliaImages/Images.jl/branch/master/graph/badge.svg)](https://codecov.io/gh/JuliaImages/Images.jl)
+[![Join the chat at https://gitter.im/JuliaImages/Images.jl](https://badges.gitter.im/JuliaImages/Images.jl.svg)](https://gitter.im/JuliaImages/Images.jl?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
+
 
 ## Documentation