WIP: add item to whats-new.rst

Author: kmuehlbauer

doc/whats-new.rst

@@ -25,6 +25,11 @@ Breaking changes
 
 New Features
 ~~~~~~~~~~~~
+- Support using an existing, opened h5netcdf ``File`` with
+  :py:class:`~xarray.backends.H5NetCDFStore`. This permits creating an
+  :py:class:`~xarray.Dataset` from a h5netcdf ``File`` that has been opened
+  using other means (:issue:`3618`).
+  By `Kai Mühlbauer <https://github.com/kmuehlbauer>`_.
 - Implement :py:func:`median` and :py:func:`nanmedian` for dask arrays. This works by rechunking
   to a single chunk along all reduction axes. (:issue:`2999`).
   By `Deepak Cherian <https://github.com/dcherian>`_.