Add note about monomorphisation

Generics with static dispatch can lead to duplicated functions, and if they are big enough, it will make for a large piece of the final binary.

Alternatives are dynamic dispatch with `dyn`, or just removing generic code.

Is seems very important for minimizing binary size.

(If you don't want to change the function API, **and** the function is being generic over something like `AsRef`, you can also create helper function which resolves `path.as_ref()` and calls a non-generic function that receives `&Path` instead.)