DOC: Add pip install odfpy for io.rst

Fixes pandas-dev#50487

Author: luke

doc/source/user_guide/io.rst

@@ -3833,7 +3833,7 @@ OpenDocument Spreadsheets
 The io methods for `Excel files`_ also support reading and writing OpenDocument spreadsheets
 using the `odfpy <https://pypi.org/project/odfpy/>`__ module. The semantics and features for reading and writing
 OpenDocument spreadsheets match what can be done for `Excel files`_ using
-``engine='odf'``.
+``engine='odf'``. ``pip install odfpy`` may be required, if is not already installed.
 
 The :func:`~pandas.read_excel` method can read OpenDocument spreadsheets