Add contribution guideline

CONTRIBUTING.md

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+# Contributing to Cal-Heatmap
+
+## Editing source file
+
+All edition should be done on `src/cal-heatmap.js`.
+
+**DO NOT** edit javascripts file located in the root, as they're built via grunt.
+
+## Building final javascripts
+
+Cal-Heatmap uses `grunt` to build final javascripts and stylesheets.
+
+Install grunt via
+
+    npm install -g grunt-cli
+
+then run
+
+    grunt build
+
+## Tests
+
+Cal-Heatmap is tested with [Qunit](https://qunitjs.com/).
+
+Tests files are located in the `test` directory, and can be run by opening the `index.html` file.
+
+If you want to add or edit existing tests, **do not** edit the `test/test.js` file. Instead, add them in `test/src`, under the corresponding module. Take a look at existing tests for reference.
+
+You can then run `grunt build` to generate the final `test/test.js` file.
+
+## Submitting changes
+
+Submit your pull requests with only files located in `src` folders. Do not commit files generated by grunt (.js located in the root and `test/test.js`).