Add a CONTRIBUTING.md file

CONTRIBUTING.md

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+# Guidelines for contributing
+
+## Code formatting
+
+We use the [Closure
+Linter](https://developers.google.com/closure/utilities/docs/linter_howto) for
+checking JavaScript files for style issues. To execute the linter use:
+
+    $ make lint
+
+This command assumes that the `gjslint` command in on your PATH.
+
+## Code compilation
+
+The OpenLayers 3 is compiled and type-checked using the [Closure
+Compiler](https://developers.google.com/closure/compiler/). To
+compile the code use:
+
+    $ make build-all
+
+## Generating doc
+
+We use [jsdoc3](https://github.com/jsdoc3/jsdoc) to generate the API
+doc. Changing the code can break the generation of the API
+doc. Use the following to generate the doc:
+
+    $ make doc
+
+This command assumes that the `jsdoc` command is on your PATH.
+
+## Tests
+
+Changes should not break existing tests. We use
+[PhantomJS](http://phantomjs.org/) to run tests *headlessly*.
+Use the following to run the tests:
+
+    $ make test
+
+This command assumes that the `phantomjs` command is on your PATH.
+
+## Commit messages
+
+We follow http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html
+for the formatting of commit messages.
+
+Basically, here's what a Git commit message should look like:
+
+    Header line: explaining the commit in one line
+
+    Body of commit message is a few lines of text, explaining things
+    in more detail, possibly giving some background about the issue
+    being fixed, etc etc.
+
+    The body of the commit message can be several paragraphs, and
+    please do proper word-wrap and keep columns shorter than about
+    74 characters or so. That way "git log" will show things
+    nicely even when it's indented.
+
+    Further paragraphs come after blank lines.