How should libraries integrate with IDO?

[Philipp Stephani]

Hi,

what should a library do that wants to use `completing-read', but
optionally support IDO?
- just call `completing-read' and tell the user to set
`completing-read-function' to `ido-completing-read'?
- provide a library-specific customization option, like
`magit-completing-read-function'?
- use `ido-completing-read' whenever ido.el is loaded?
- or something else?

If this isn't documented yet in the ELisp or IDO manuals, please document
it there.

Thanks,
Philipp