Add comment about dependents on the internal structure of Calculus.jl

This was my answer to https://github.com/johnmyleswhite/Calculus.jl/issues/34 before it was opened.

Other packages starts to depend on the internal structure of this package. I think it is reasonable to add a comment to tell future developers that might want to change the internal structure which public packages they will break, so that they can ping the authors. 
See: https://github.com/scidom/DualNumbers.jl/pull/1