We need a contributing.md document

[jwdebelius]

As we start to expand analyses and contributors, we need contributing guidelines. These probably need to cover the following areas:

• Requirements for testing code (i.e. separate logic and plotting, plotting does not need to be tested)
• What data can be hosted on GitHub, and where alternative data should be hosted
• Structure for IPython Notebooks
• Whether IPython Notebooks represent a tutorial of how data was generated, or if they need to reflect all data generated.