Add documentation for updating the package

[joshuadavidthomas]

Since this package is designed to be used in a distributed fashion and involves models and the potential for migrations, we should document how to go about updating the package.

Any changes to the models that will involve migrations should be split across (at least) two releases:

1. an initial release that contains the changes in a backwards compatible way and a deprecation warning
2. a major release bump along with removing the deprecated old code.

IMO this is unlikely to happen as I think the models are pretty stable, however it should be documented now as to get ahead of it just in case. May be a bit of YAGNI, but better to do it than not.

[joshuadavidthomas]

The order of updating is important too. I think the recommendation should be that the Docker image or Django app running the relay service should be updated first, then any Django projects using the relay service as an email backend should follow shortly afterwards.