Add explanations or better support for testing

[craiglabenz]

Because classes like Query, CollectionReference, and DocumentReference are base/final, they cannot be implemented within application code. This complicates using pkg:mocktail or pgk:mockito to write tests for the code which immediately invokes functions from this library.

It would be nice to either include useful fakes in the library, or, if there is already a way application authors can easily write tests, to add a section explaining that to the documentation.

Much love!

[Ehesp]

Good shout!