Fix/Update Contribution instructions

[abandoned-prototype]

The "submitting a PR" piece of our contribution instructions needs some updates: https://github.com/lucyparsons/OpenOversight/blob/develop/CONTRIB.md#submitting-a-pr, as it currently gives incorrect instructions. Specifically we need to make it clear that contributors need to create their own fork of the project in order to be able to push their branches onto github and create a PR from there. In addition, linking to this guide might be useful: https://guides.github.com/activities/forking/