message: add ring example PDB IDs
diff: * ACE (many, e.g. 5AGU, 1E1X)
  * ACY in 7ABY
  * NH2 (many, e.g. 6Y13)
  +* Ligands with many rings
  + * STU (e.g. 1U59) - many fused rings
  + * HT (e.g. 127D) - rings connected by a single bond
  + * J2C (e.g. 7EFJ) - rings connected by a single atom
  + * RBF (e.g. 7QF2) - three linearly fused rings
  + * TA1 (e.g. 1JFF) - many fused rings (incl. a 8-member rings)
  + * BPA (e.g. 1JDG) - many fused rings
  + * CLR (e.g. 3GKI) - four fused rings
  Assembly symmetries
  * 5M30 (Assembly 1, C3 local and pseudo)
  