Add #451 to release notes

Author: GamzeUnlu95

RELEASE_NOTES.rst

@@ -30,6 +30,13 @@ All changes
 
 - Some MESSAGEix :doc:`tutorials <tutorials>` are runnable with the :class:`.IXMP4Backend` introduced in :mod:`ixmp` version 3.11 (:pull:`894`, :pull:`941`).
   See `Support roadmap for ixmp4 <https://github.com/iiasa/message_ix/discussions/939>`__ for details.
+- Extend the auxiliary variable :ref:`_commodity_balance` to include input and output flows based on CAP and CAP_NEW variables (:pull:`451`).
+
+  When loading a Scenario created with a version of `message_ix` older than 3.11.0,
+  these items will be initialized (and left empty),
+  using at most one call to :meth:`~message_ix.Scenario.commit`.
+  See :meth:`.MESSAGE.initialize`.
+
 - Adjust use of :ref:`type_tec <mapping-sets>` in :ref:`equation_emission_equivalence` (:pull:`930`, :issue:`929`, :pull:`935`).
 
   This change reduces the size of the ``EMISS`` variable,