message: add interesting pdb entry
diff: * DZ has C1 instead of N1 (e.g. 6I4N)
  * DP has N5 instead of C5 and C7 instead of N7 (e.g. 6I4N)
  * Beta & Gamma peptides (e.g. 1GAC, 6PQF)
  +* Helices of D-amino acids (e.g. 7QDI)
  * Mixed (heterogeneous) all-atom/trace-only RNA model (1JGQ)
  * Polymers with residues with missing trace atoms (e.g. 2QFJ)
  * Modified RNA bases (1y26, 5L4O)
  