Added instructions on how to start contribute (jtblin#629)

rootux · jtblin · commit af7a58ec39b6 · 2017-05-29T19:25:51.000+10:00
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
@@ -3,6 +3,7 @@
 1. Create an issue
 1. Fork the repo
 1. Install dependencies: `npm install` and `bower install`
+1. `npm run docker-build` to build the docker image
 1. Make your changes
 1. Install [GraphicsMagick](http://www.graphicsmagick.org/)
 1. Run linter and tests: `gulp check`
