Document/consider schema migration concepts

[dw]

Boils down to:

1. Removal of a field from meta.py does not automatically remove the underlying data. Re-adding the field will cause the old data to reappear. Need a method to truly orphan a particular field ID.
2. structlib will default to preserving unknown field IDs, since that allows in-place upgrades of an application. Some mode might be useful that causes unknown IDs to be discarded.
3. Field renames currently cause very strange behaviour. Perhaps add a Field() parameter such as "internal_name=" to mitigate this.
4. Adding indices is currently hard (e.g. ticket #15)

etc etc