Document the type registration system

[muhrin]

Currently, mincePy is conservative with discovering compatible types, essentially relying on manual register_type() calls, or assuming that when the users calls save() that they want to register that type helper if it is not already registered. There is a plugin system that uses setup.py endpoints to automatically discover types but this is entirely undocumented.

The documentation should be extended to include:

• Information on when types will be registered (and when to manually register)
• An explanation of why the historian adopts the behaviour it does
• Documentation of the type discovery endpoint hooks
• Advice on what to use as TYPE_IDs and in which situations