add note to the top of the release page noting the plan for 2.18.* and 3.0 (#1816)

Author: jhamman

docs/release.rst

@@ -13,6 +13,11 @@ Release notes
     # to document your changes. On releases it will be
     # re-indented so that it does not show up in the notes.
 
+.. note::
+   Zarr-Python 2.18.* is expected be the final release in the 2.* series. Work on Zarr-Python 3.0 is underway.
+   See `GH1777 <https://github.com/zarr-developers/zarr-python/issues/1777>`_ for more details on the upcoming
+   3.0 release.
+
 .. _unreleased:
 
 Unreleased