downstream packages need to put `Arrow.ArrowTypes.registertype!` statements in `__init__`

[ericphanson]

Otherwise Arrow.ArrowTypes.ARROW_TO_JULIA_TYPE_MAPPING doesn't get updated on package load, since the registertype! call gets executed at precompile time and that state is not preserved for using time. It seems like this is just a consequence of the design choice to use a global dictionary instead of dispatch (like StructTypes does). This caught me off-guard so I figured I'd file an issue; I think it should be mentioned in the docs at least.