Add tutorial on implementing extensions in PySTAC.

docs/tutorials.rst

@@ -26,3 +26,12 @@ How to create STAC Catalogs with PySTAC
 - :ref:`Docs version </tutorials/how-to-create-stac-catalogs.ipynb>`
 
 This was a tutorial that was part of a 30 minute presentation at the `community STAC sprint <https://github.com/radiantearth/community-sprints/tree/master/11052019-arlignton-va>`_ in Arlington, VA in November 2019. It runs through creating a STAC of image or label items from the `SpaceNet 5 <https://www.topcoder.com/challenges/30099956>`_ dataset.
+
+Adding New and Custom Extensions
+--------------------------------
+
+- :tutorial:`GitHub version <how-to-create-stac-catalogs.ipynb>`
+- :ref:`Docs version </tutorials/adding-new-and-custom-extensions.ipynb>`
+
+This tutorial goes over how to contribute new extensions to PySTAC as well as how to implement
+your own custom extensions.