Unclear that indices 0-9 already registered for primitives

[cypherdare]

There is no way short of reading the source code that I've found for a user to know that Kryo has already used indices 0-9 to register all the primitives and String, and so can accidentally overwrite those registrations if explicitly setting the ID with kryo.register(class, id). A typical user (if I'm one) starts registering classes at index 0 or 1 and up from there, thereby wiping out the registrations of all the primitives.

I'm not sure if there's a clean solution. It might be adequate to update the documentation to recommend assigning with indices >= 10. But when registration is set to required, and in light of #398, enforcement might be necessary.

[magro]

Sounds reasonable.

@NathanSweet WDYT?

[magro]

@NathanSweet WDYT about this?

[magro]

Closed with #509

[NathanSweet]

Sorry I'm late. Updating the docs in #509 is good.

Note that a user can clear and re-register all classes to have control of the ordinals.