Add a release note for _Graph/Model changes

releasenotes/notes/_Graph-e85b234643bb01a7.yaml

@@ -0,0 +1,14 @@
+---
+features:
+  - |
+    Add a new Cython extension type, ``_Graph``, in a new ``dwave.optimization._graph``
+    submodule. The ``_Graph`` class is responsible for managing a C++
+    ``dwave::optimization::Graph``.
+    Also make ``Model`` a Python class that inherits from ``_Graph``, rather than
+    a Cython extension type.
+upgrade:
+  - |
+    It is no longer possible to ``from dwave.optimization.model cimport Model``.
+    Instead, importers should ``from dwave.optimization._graph cimport _Graph``.
+    A ``_Graph`` has most of the functionality that a ``Model`` did, although
+    most of the symbol construction methods have been kept on ``Model``.