added interesting pdb entry

docs/interesting-pdb-entries.md

@@ -26,3 +26,4 @@
     * Protein (1BRR, 5Z6Y)
     * DNA (5D3G)
 * Multiple models with different sets of ligands or missing ligands (1J6T, 1VRC, 2ICY, 1O2F)
+* Long linear sugar chain (4HG6)