Add development documentation on service properties migration

In https://gitlab.com/gitlab-org/gitlab-ce/issues/43942#note_147395448, I mentioned:

1. Plan will migrate the issue tracker services (including Jira) to their own tables in 11.10.
2. We'll create issues based on service type for other teams to take after 12.0.
3. Our work will be as reusable as possible.
4. We'll hopefully be done in 11.11 or 12.0 (as these involve background migrations, they aren't 'done' until at least one release after they are introduced).

Obviously I was too optimistic in the timeframe. So we should add development documentation to explain the state to other engineers, so that people know not to add too much complication to existing services. (A new field is generally fine as it would need to be migrated anyway, but a change to field handling in general is a harder case to make.)

This documentation should say:

1. Why we're doing this.
2. How to do this for your service.
3. How to handle encrypted fields.