Update release notes

Author: fredericDelaporte

releasenotes.txt

@@ -3,7 +3,7 @@ Build 5.2.0
 
 Release notes - NHibernate - Version 5.2.0
 
-120 issues were resolved in this release.
+121 issues were resolved in this release.
 
     ##### Possible Breaking Changes #####
         * With PostgreSQL, an HQL query using the bitwise xor operator "^" or "bxor"
@@ -127,6 +127,7 @@ Not yet merged and not counted: #1859, #1855, #1836, #1831, #1793, #1812, #1808,
 
 ** Improvement
 
+   * #1869 Replace an O(n) lookup in LINQ query parsing by an O(1) one
    * #1846 Remove dependency on System.Security.Permissions package for .NET Standard and .NET Core
    * #1842 Remove WcfOperationSessionContext from .Net Core and .Net Standard
    * #1838 Cannot add HqlJoin to HqlFrom