Clarify Foreign Keys

[scott-w]

In the README, we should clarify how ForeignKey relationships work in the context of SQL Views.

[ktosiek]

Does it make sense to use FKs with on_delete different than DO_NOTHING? If not, would a system check for that make sense?

[scott-w]

In theory, I believe you can write SQL that manages the update and delete cases to map them through. It does feel like an edge case, however, so I'd be happy to change the default to models.DO_NOTHING somehow.