Add STAC tutorial to tutorials page.

Also add link to SpaceNet 5 challenge.

Author: lossyrob

docs/tutorials.rst

@@ -1,13 +1,18 @@
 Tutorials
 #########
 
+The tutorials below are available on GitHub: https://github.com/azavea/pystac/tree/develop/docs/tutorials
+
 PySTAC Introduction
 -------------------
 
 :ref:`This notebook </tutorials/pystac-introduction.ipynb>` gives an introduction to PySTAC concepts through code examples.
 
-
 PySTAC SpaceNet tutorial
 ------------------------
 
 :ref:`This notebook </tutorials/pystac-spacenet-tutorial.ipynb>` shows how to create and manipulate a STAC of `SpaceNet <https://spacenetchallenge.github.io/>`_ data.
+
+How to create STAC Catalogs with PySTAC
+---------------------------------------
+:ref:`This notebook </tutorials/how-to-create-stac-catalogs.ipynb>` is 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.

docs/tutorials/how-to-create-stac-catalogs.ipynb

@@ -60,7 +60,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "To give us some material to work with, lets download a single image from the Spacenet 5 challenge. We'll use a temporary directory to save off our single-item STAC."
+    "To give us some material to work with, lets download a single image from the [Spacenet 5 challenge](https://www.topcoder.com/challenges/30099956). We'll use a temporary directory to save off our single-item STAC."
    ]
   },
   {
@@ -704,7 +704,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "Beyond what a Catalog requires, a Collection requires a license, and an `Extent` that describes the range of space and time that the items it hold occupy."
+    "Beyond what a Catalog reqiures, a Collection requires a license, and an `Extent` that describes the range of space and time that the items it hold occupy."
    ]
   },
   {