Added a CONTRIBUTING.md file outlining how to setup the project and c…

…ontribute changes.

CONTRIBUTING.md

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+# Contributing
+
+Want to contribute a change to Snazzy Info Window or fix an issue? Great! Follow
+the steps below to setup your environment, make your change, and create a pull
+request.
+
+## Getting Started
+
+You can clone this repository locally from GitHub by running this command
+in your command line:
+
+    git clone https://github.com/atmist/snazzy-info-window.git
+
+If you want to make contributions to the project, [forking the project][fork]
+is the easiest way to do this. You can then clone down your fork instead:
+
+    git clone https://github.com/MY-USERNAME-HERE/snazzy-info-window.git
+
+[fork]: https://help.github.com/articles/fork-a-repo/
+
+## Required Libraries
+
+- npm
+- [EditorConfig][editor config] plugin for your editor of choice
+
+[editor config]: http://editorconfig.org/
+
+## Making Changes
+
+Now that you have the code you can get it running locally. The project is
+written in ES6 and transpiled into JavaScript with Babel so a build step is
+required before using the project in the browser.
+
+Install development dependencies:
+
+    npm install
+
+Run the build for testing purposes:
+
+    gulp build:test
+
+This will create the necessary files in the test directory. You can then open
+`./test/index.html` or `./test/multi.html` for testing the plugin on a map.
+Now it's time to make your change! Run `gulp help` to see other
+available commands.
+
+Run a watch:
+
+    gulp watch
+
+Make the necessary code changes. Every time you save a file, it will trigger
+a build of the JavaScript and SCSS files and lint everything according to our
+style guidelines below. You can then just refresh the browser to see your
+change.
+
+Once you've made your changes, write a good commit message, push to your fork,
+and submit a [pull request][pull request]. We will respond as soon as possible.
+
+[pull request]: https://help.github.com/articles/about-pull-requests/
+
+## Style Guidelines
+
+To maintain consistent code style between developers we lint both the
+JavaScript and SCSS code in the project using ESLint and SASSLint.
+
+For JavaScript we use the `airbnb-base` rules for ESLint with a
+few customizations outlined in the `.eslintrc` file. SASS Lint has their own
+SCSS rules with our customizations outlined in `.sass-lint.yml` file.