Update 0.9.0-notes.rst

add PR to release notes

Author: jcapriot

docs/release/0.9.0-notes.rst

@@ -71,4 +71,5 @@ Pull requests
 * `#320 <https://github.com/simpeg/discretize/pull/320>`__: Nodal boundary example
 * `#321 <https://github.com/simpeg/discretize/pull/321>`__: Cyl mesh generalization
 * `#322 <https://github.com/simpeg/discretize/pull/322>`__: Capture scroll
-* `#323 <https://github.com/simpeg/discretize/pull/323>`__: 0.9.0 release
+* `#323 <https://github.com/simpeg/discretize/pull/323>`__: 0.9.0 release
+* `#324 <https://github.com/simpeg/discretize/pull/324>`__: Add total_nodes method to TreeMeshes