📝 Add a change note for PR aio-libs#963

CHANGES/963.packaging.rst

@@ -0,0 +1,11 @@
+The leading ``--`` has been dropped from the :pep:`517` in-tree build
+backend config setting names. ``--pure-python`` is now just ``pure-python``
+-- by :user:`webknjaz`.
+
+The usage now looks as follows:
+
+.. code-block:: console
+
+    $ python -m build \
+        --config-setting=pure-python=true \
+        --config-setting=with-cython-tracing=true