Add documentation on running tests with in-memory databases

[schambers]

Moved from fluentmigrator as this is a valid documentation request originally submitted by @CraigComeOnThisNameCantBeTaken

Is your feature request related to a problem? Please describe.
There is no documentation on how to use fluentmigrator with inmemory tests in the same style as EF.

Describe the solution you'd like
Documentation should be added.

Additional context
I have implemented this myself. Details for which can be found in my answer on stackoverflow here:
https://stackoverflow.com/questions/53282474/how-to-test-fluent-migrations-with-an-in-process-migration-runner-and-a-in-memor/66926940#66926940

Is this a feature you'd like to submit a PR for?
I can if this proves a popular idea.