Document what should live here, vs in mathlib

[414owen]

Coming from Haskell, I would have expected Bifunctor to be here.
Is there a set of rules, for deciding where things should live?
I'm trying to avoid importing mathlib, at the moment, because I'm working on programming, rather than theorem proving.

[fgdorais]

There is no strict line at this time. If you need something for programming purposes, then it should probably be here or in core.