DOC: Add numpngw to third party packages.

Author: WarrenWeckesser

doc/_static/numpngw_animated_example.png

@@ -235,3 +235,14 @@ border, and logo.
 
 .. image:: /_static/mpl_template_example.png
     :height: 330px
+
+numpngw
+=======
+
+`numpngw <https://pypi.org/project/numpngw/>`_  provides functions for writing
+NumPy arrays to PNG and animated PNG files.  It also includes the class
+``AnimatedPNGWriter`` that can be used to save a Matplotlib animation as an
+animated PNG file.  See the example on the PyPI page or at the ``numpngw``
+`github repository <https://github.com/WarrenWeckesser/numpngw>`_.
+
+.. image:: /_static/numpngw_animated_example.png