Add doc for creating new adapters

[TheSmartnik]

Following #628

[TheSmartnik]

@botandrose I've added WIP doc as you asked me in #628 (comment)

I haven't managed to follow it fully since 2.0.0 is not yet published

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[botandrose]

@TheSmartnik This is really great! Thank you! I'll be doing some more refactoring and test improvement groundwork in a few PRs over the next few days, and I expect I'll merge this after that. Having a documented baseline for where the ORM adapter interface is right now is going to be extremely useful for evaluating future PRs that attempt to improve it! I'm expecting we can simplify it quite a bit before v2.0.

[TheSmartnik]

@botandrose awesome, glad you liked it :)

[botandrose]

Thanks again, @TheSmartnik!

Since we're not quite ready for people to actually follow these instructions, I'm going to split this content out into its own file, with a big warning up top saying as much. Once v2.0.0 is out the door, it will be restored to the README.