Contributing to Chado documentation

[spficklin]

At least for the sake of the PMC I propose we need a document about how to contribute documentation and schema changes to Chado. I propose we add

1. How to edit documentation: this would cover things like how to setup Sphynx, how to compile the docs for local viewing.
2. How to write Flyway migrations: perhaps move the section from the Flyway documentation about creating new PRs into this document.