add release notes

Author: RobbieKiwi

doc/release_notes.rst

@@ -3,6 +3,7 @@ Release Notes
 
 .. Upcoming Version
 
+* Add ``mock_solve`` option to model.solve for quick testing without actual solving
 * Fix compatibility for xpress versions below 9.6 (regression)
 * Performance: Up to 50x faster ``repr()`` for variables/constraints via O(log n) label lookup and direct numpy indexing
 * Performance: Up to 46x faster ``ncons`` property by replacing ``.flat.labels.unique()`` with direct counting