Clarify library goals

[matthewfeickert]

As discussed in the Scikit-HEP Gitter it is still unclear to me if a library like this with a higher level API that wraps common patterns of the Scikit-HEP ecosystem are actually useful or if they are community anti-patterns. This should be followed up and fleshed out.

It is clear though that the goals of heputils are different from scikit-hep (the metapackage library).