Imrprove changelog

Author: alex-kulakov

ChangeLog/6.0.13_dev.txt

@@ -4,4 +4,5 @@
 [main] Join/LeftJoin is denied to have the same expression instance for both inner/outer selector
 [main] Addressed issue when wrong type of join was chosen when .First/FirstOrDefalult() method was used as subquery
 [main] Added dedicated exception when RenameFieldHint.TargetType exists in current model but absent in storage model
+[main] Xtensive.Sql.Model.NodeCollection<T>.Add() throws understandable exception in case of duplicate name of item
 [postgresql] Fixed issue of incorrect translation of contitional expressions including comparison with nullable fields