Add #981 to doc/whatsnew

Author: khaeru

RELEASE_NOTES.rst

@@ -48,6 +48,8 @@ All changes
   - :mod:`message_ix.message_macro` includes :class:`.MESSAGE_MACRO`.
 
 - Document the :ref:`minimum version of Java <install-java>` required for :class:`ixmp.JDBCBackend <ixmp.backend.jdbc.JDBCBackend>` (:pull:`962`).
+- Document :ref:`how to run a local PostgreSQL instance <install-postgres>`
+  for local testing using :class:`ixmp.IXMP4Backend <ixmp.backend.ixmp4.IXMP4Backend>` (:pull:`981`).
 - Improve type hinting (:pull:`963`).
 - Fix capitalization in auxiliary_settings.gms to enable GDX output file compression on MacOS and Linux. (:pull:`965`)