[Closes #285] CONTRIBUTING.md

CONTRIBUTING.md

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+# Contribute
+
+Before anything else... Thank you for your time and contributions!
+
+## Issues (New Features, Rules & Bug Reports)
+
+If you have a feature or rule you want to see added to Elvis, please create an
+[issue](https://github.com/inaka/elvis/issues) before doing any work, so that we can discuss it and figure
+out what to do about it.
+
+In the case of rules, keep in mind you can always create your [User Defined Rules](https://github.com/inaka/elvis#user-defined-rules), in order to run them locally.
+
+When you find a bug in one of Elvis's rules or the way Elvis itself work, please provide detailed steps
+that show how to reproduce the problem.
+
+## Rules
+
+When an issue is already marked with the `rule` label and you are interested in working on it, first check
+there is no existing [branch](https://github.com/inaka/elvis/branches) with the issue's number, If so, you
+can go ahead and start working on it.
+
+The following is a checklist you can follow when implementing a new Elvis rule:
+
+- [ ] Write both test cases that fail and that pass the rule.
+- [ ] Edit the [Rules](https://github.com/inaka/elvis/wiki/Rules) wiki page, by adding the rule to the list.
+- [ ] When the issue has the label `default rule`, add the rule to [Default Rules in Elvis Webhook](https://github.com/inaka/elvis/wiki/Default-Rules-in-Elvis-Webhook) and in the [`config/elvis.config`](https://github.com/inaka/elvis/blob/master/config/elvis.config) file.
+
+## Questions?
+
+If you have any questions or general comments regarding how to contribute, please use our public [hipchat room](http://inaka.net/hipchat).