Added CONTRIBUTING.md.

CONTRIBUTING.md

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+Contributing
+============
+
+Contributions are welcome!
+
+## Bug Reports
+
+For bug reports or requests please [submit an issue](https://github.com/donnemartin/coding-challenges/issues) or send a [pull request](#pull-requests).
+
+## Notebook Installation
+
+Refer to [Accessing the Challenges](https://github.com/donnemartin/coding-challenges#accessing-the-challenges), [Notebook Installation](https://github.com/donnemartin/coding-challenges#notebook-installation), and [Nose Installation](https://github.com/donnemartin/coding-challenges#nose-installation) to set up your local environment.
+
+## Pull Requests
+
+**Please follow the general [Repo Structure](https://github.com/donnemartin/coding-challenges#repo-structure) and [Notebook Structure](https://github.com/donnemartin/coding-challenges#notebook-structure) before submitting your pull request.**
+
+### Adding Solutions to Existing Notebooks
+
+If adding new solutions to existing notebooks:
+* Append your algorithm and code solution rather than replace a fellow contributor's work
+* Verify your code passes the unit tests
+
+### Adding New Challenges
+
+There are several sample coding challenges that are not yet implemented such as the examples below.
+* These challenges are merely suggestions; feel free to send pull requests on different challenges, or even entire categories of challenges
+* Try to keep the "Challenge" title relatively short and expand on the details within the notebook
+* Check out the [template challenge and solution notebooks](https://github.com/donnemartin/coding-challenges/templates/) to help you get started
+
+| Challenge | Static Notebooks |
+|--------------------------------------------------------------------------------------------------------------|--------------------------------------------------------------------------------------------------------------------------------------------|
+| Check if a number is prime | [Contribute](https://github.com/donnemartin/coding-challenges#contributing)│[Contribute](https://github.com/donnemartin/coding-challenges#contributing) |
+| Generate a list of primes | [Contribute](https://github.com/donnemartin/coding-challenges#contributing)│[Contribute](https://github.com/donnemartin/coding-challenges#contributing) |
+
+Update the list of challenges in the README so we can enjoy your challenge!
+
+### Contributions in Languages Other than Python
+
+Feel free to share your thoughts on how we could best approach adding challenges in other [supported languages](https://github.com/ipython/ipython/wiki/IPython-kernels-for-other-languages):
+* Repo structure
+* README static notebook links
+* Installation instructions
+* Etc
+
+### Style
+
+Please refer to the style of existing notebooks to help maintain consistency.  A consistently themed collection of notebooks might help users more seamlessly transition from challenge to challenge.
+
+Review [PEP-8](https://www.python.org/dev/peps/pep-0008/) and the following [style guide](https://google-styleguide.googlecode.com/svn/trunk/pyguide.html).
+
+### Notebook Author(s)
+
+When contributing to a notebook, update the notebook author(s):
+
+<small><i>This notebook was prepared by [Joe](https://github.com/Joe) and [Jane](https://github.com/Jane). Source and license info is on [GitHub](https://github.com/donnemartin/coding-challenges).</i></small>