Add migration guide for integer PK field

[florimondmanca]

Is your feature request related to a problem? Please describe.
#40 (Integer primary key field) got merged, but it includes a breaking change for users that have models that reference APIKey as a foreign key, e.g.:

class Person(models.Model):
    api_key = models.ForeignKey("APIKey")

A proper migration guide for users is a prerequisite for 2.0.

Describe the solution you'd like
Add a migration guide to a new docs/ folder. It should explain:

• What the change is
• How to check if one is affected
• Steps to securely upgrade without losing existing data

This guide will be referenced in the changelog when preparing the 2.0 release.

Many hints were already provided by @eagle-r in #41, especially here: #41 (comment)

Another detailed plan for migrating FKs was shared here: #42 (comment)

Describe alternatives you've considered
/

Additional context
Please read #41 for the initial full conversation.

[florimondmanca]

I’ve just brought the changes from master back onto dev/2.0. The tests were already using the abstract model which means that the derived model did not benefit from the migrations from #41 right away — I had to edit the migration to basically replicate the one from #41. We should also document this here.

[florimondmanca]

Closing as this issue (and the dev/2.0 branch…) is now very out of date with master.