Added instructions on how to contribute

CONTRIBUTING.md

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+# How to contribute
+
+We're always happy about useful new pull requests. Keep in mind that the better
+your pull request is, the easier it can be added to `xml2js`. As such please
+make sure your patch is ok:
+
+ * `xml2js` is written in CoffeeScript. Please don't send patches to
+   the JavaScript source, as it get's overwritten by the CoffeeScript
+   compiler. The reason we have the JS code in the repository is for easier
+   use with eg. `git submodule`
+ * Make sure that the unit tests still all pass. Failing unit tests mean that
+   someone *will* run into a bug, if we accept your pull request.
+ * Please, add a unit test with your pull request, to show what was broken and
+   is now fixed or what was impossible and now works due to your new code.
+ * If you add a new feature, please add some documentation that it exists.
+
+If you like, you can add yourself in the `package.json` as contributor if you
+deem your contribution significant enough. Otherwise, we will decide and maybe
+add you.