Release notes for 2025.09.1 (#10797)

* release notes for 2025.09.1

* elaborate

Author: shoyer

doc/whats-new.rst

@@ -7,8 +7,16 @@ What's New
 
 .. _whats-new.2025.09.1:
 
-v2025.09.1 (unreleased)
------------------------
+v2025.09.1 (September 29, 2025)
+-------------------------------
+
+This release contains improvements to netCDF IO and the
+:py:func:`DataTree.from_dict` constructor, as well as a variety of bug fixes.
+In particular, the default netCDF backend has switched from netCDF4 to h5netcdf,
+which is typically faster.
+
+Thanks to the 17 contributors to this release:
+Claude, Deepak Cherian, Dimitri Papadopoulos Orfanos, Dylan H. Morris, Emmanuel Mathot, Ian Hunt-Isaak, Joren Hammudoglu, Julia Signell, Justus Magin, Maximilian Roos, Nick Hodgskin, Spencer Clark, Stephan Hoyer, Tom Nicholas, gronniger, joseph nowak and pierre-manchon
 
 New Features
 ~~~~~~~~~~~~