update whats-new.rst [skip-ci]

Author: keewis

docs/whats-new.rst

@@ -25,6 +25,7 @@ What's new
   By `Mika Pflüger <https://github.com/mikapfl>`_.
 - implement :py:meth:`Dataset.pint.sel` and :py:meth:`DataArray.pint.sel` (:pull:`60`).
   By `Justus Magin <https://github.com/keewis>`_.
+- implement :py:attr:`Dataset.pint.loc` and :py:meth:`DataArray.pint.loc` (:pull:`79`).
 - implement :py:meth:`Dataset.pint.drop_sel` and :py:meth:`DataArray.pint.drop_sel` (:pull:`73`).
   By `Justus Magin <https://github.com/keewis>`_.
 - implement :py:meth:`Dataset.pint.reindex`, :py:meth:`Dataset.pint.reindex_like`,