add interesting pdb entries info

docs/interesting-pdb-entries.md

+* Cyclic polymers (1sfi, 6dny, 1HVZ)
+* B-DNA (1bna)
+* Missing carbonyl oxygen (1gfl)
+* Mono-saccharides with alt locs (1B5F)
+* Microheterogeneity (1EJG, 3NIR)
+* PNA: peptide nucleic acid (5eme, 1xj9)
+* Peptide derived residues
+    * GFP chromophores (5Z6Y)
+* Nucleotides that don’t have a parent base set, i.e. detect purine/pyrimidine from geometry (THX in 1AUL, OMC in e.g. 5D3G)
+* Bases with modified ring atoms
+    * DZ has C1 instead of N1 (e.g. 5I4N)
+    * DP has N5 instead of C5 and C7 instead of N7 (e.g. 5I4N)
+* Beta & Gamma peptides (e.g. 1GAC, 6PQF)
+* Mixed (heterogeneous) all-atom/trace-only RNA model (1JGQ)
+* Polymers with residues with missing trace atoms (e.g. 2QFJ)
+* Modified RNA bases (1y26, 5L4O)
+* Discontinuous chains, i.e. gaps in the sequence (3sn6)
+* Lots of sheets (1cbs)
+* DNA (2np2, 1d66)
+* C-alpha only (2rcj)
+* Not cyclic, but termini are backbone-only and within distance but seqIds are not compatible (6SW3)
+* Close backbone atoms but not linked (e.g. 4HIV)
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