Add release note

releasenotes/notes/custom-iterators-dce8557a8f87e8c0.yaml

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+---
+features:
+  - |
+    Rustworkx functions that return custom iterable objects, such as
+    :meth:`.PyDiGraph.node_indices`, now have an associated custom iterator object for these.  This
+    provides a speedup of approximately 40% for iterating through the custom iterables.
+
+    These types are not directly nameable or constructable from Python space, and other than the
+    performance improvement, the behavior should largely not be noticable from Python space.