Document the limitation of the PDB and PDBx parsers

[jbarnoud]

I discussed with @alexdb27 last week and he expressed concerns that users could use PBxplore on strange PDB inputs without taking the parsers limitation into account. This would lead to odd results citing PBxplore.

One way to prevent, or at least mitigate, this would be to document that the PDB and PDBx parsers do not handle all the oddities that PDB files can contain.