Add contributing instructions

CONTRIBUTING.md

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+# Contributing to Learn.co Curriculum
+
+We're really exited that you're about to contribute to the [open curriculum](https://learn.co/content-license) on [Learn.co](https://learn.co). If this is your first time contributing, please continue reading to learn how to make the most meaningful and useful impact possible.
+
+## Raising an Issue to Encourage a Contribution
+
+If you notice a problem with the curriculum that you believe needs improvement
+but you're unable to make the change yourself, you should raise a Github issue
+containing a clear description of the problem. Include relevant snippets of
+the content and/or screenshots if applicable. Curriculum owners regularly review
+issue lists and your issue will be prioritized and addressed as appropriate.
+
+## Submitting a Pull Request to Suggest an Improvement
+
+If you see an opportunity for improvement and can make the change yourself go
+ahead and use a typical git workflow to make it happen:
+
+* Fork this curriculum repository
+* Make the change on your fork, with descriptive commits in the standard format
+* Open a Pull Request against this repo
+
+A curriculum owner will review your change and approve or comment on it in due
+course.
+
+# Why Contribute?
+
+Curriculum on Learn is publicly and freely available under Learn's
+[Educational Content License](https://learn.co/content-license). By
+embracing an open-source contribution model, our goal is for the curriculum
+on Learn to become, in time, the best educational content the world has
+ever seen.
+
+We need help from the community of Learners to maintain and improve the
+educational content. Everything from fixing typos, to correcting
+out-dated information, to improving exposition, to adding better examples,
+to fixing tests—all contributions to making the curriculum more effective are
+welcome.