Update and finalize data contribution docs

[jenniferthompson]

Leaving this here mostly as a to-do list.

• Add link to best practices from data.world (https://docs.google.com/document/d/1p5A2DQ5gFC7XVKNVDw_ifKnycv_j1udmqY1M0rjbcxo/edit)
• Clarify what should go in README vs what should go in data dictionary
• ~Add link to objectives statement~
• Add preference for CSV + feather formats unless there's a reason to do otherwise
• ~Edit folder to datadictionaries once reorg PR is merged~
• Clarify that one does not simply upload into data.world (must request contributor invite)

[jenniferthompson]

done!