add notes on developing with the professional drivers

[simonpcouch]

Bringing together some information on the Posit Professional Drivers I've pulled together the last few days.

The big picture:

• In general, the professional drivers themselves can be easily installed through Desktop Pro, which is available to Posit employees.
• The professional drivers are tested using (for the most part, publicly available) containerized databases, and that tooling on macOS is somewhere between just works and a good place to start, depending on the database.
• Oracle is still a huge pain.

Thank you for the help, @bschwedler! Feedback welcome.