Document use of `rc` to add pseudo-atoms in `Molecule`s about porousmaterials.jl

[eahenle]

We don't include anything like :VOID in the mass list, because some people call it that, others call it :X, still others might call it :None, etc. The user should set whichever one of these they intend to use by adding it to the (exported, global) rc dictionary before calling Molecule:

rc[:atomic_masses][:VOID] = 0.

[qlx17]

Thanks for your explanation. That is a more general way to add custom atomic masses. Perhaps this should be documented in the molecules chapter of the documentation?

[eahenle]

Yes, that is a very good idea.