Add description of how to update custom claims on refresh to README

[henrywoody]

Adds a description of how to update custom claims on token refresh requests. This expands the section of the README called "Customizing token claims" and the added description's structure mirrors that of the existing "Customizing token claims" section.

This is helpful to include because it is not uncommon for token data to become outdated quicker than refresh tokens expire on average. Issue 33 was a question about this sort of behavior and this added description borrows from the response to issue 33 by davesque. Similarly, issue 59 (about revoking refresh tokens if the user's password is changed) would benefit from this description.

[henrywoody]

I think the test failure is coming from a dependency since I only changed the README.

Error Message:

django.db.utils.NotSupportedError: SQLite schema editor cannot be used while foreign key
constraint checks are enabled. Make sure to disable them before entering a transaction.atomic()
context because SQLite3 does not support disabling them in the middle of a multi-statement
transaction.