Added documentation explaining contrib and outlining process around it

[reinecke]

Adds a README.md to the contrib area to explain the break-out process in-progress.

[jminor]

This is great! Could you put a link to it in place of the "Registering Your Contrib Adapter" here: https://github.com/AcademySoftwareFoundation/OpenTimelineIO/blob/main/docs/tutorials/write-an-adapter.md

[reinecke]

@jminor good note on the other adapter page - I opted to reproduce the instructions a bit there so when the contrib area migration is done we won't have to update any links over there. I also reshuffled/touched it up so that it highlighted the preferred route of cloning the template repo.