Update documentation

docs/examples.rst

@@ -67,3 +67,35 @@ scene with the Roman WFI F129 filter offset by 0.5 degrees in RA and rotated by
 
 In this case, the output catalog(s) will show the actual applied count rates. Whether there 
 is only one output catalog or two depends on the input catalog format.
+
+
+Fast Extended Sources
+----------------------
+
+In STIPS 2.2 we included an option to inject extended sources in scenes using a sersic-profile
+approximation, which is less accurate than the current implementation, but ~8 times faster.
+
+To activate this feature, users must turn on the ``fast_galaxy`` flag. This is how the syntax 
+looks, starting from the examples listed in the :doc:`STIPS Basic Tutorial <basic_tutorial>`.
+
+.. code-block:: python
+
+    observation_parameters = {
+                              'instrument': 'WFI',
+                              'filters': ['F129'],
+                              'detectors': 1,
+                              'distortion': False,
+                              'background': 0.15,
+                              'fast_galaxy': True,
+                              'observations_id': 1,
+                              'exptime': 1000,
+                              'offsets': [offset]
+                              }
+
+.. note::
+
+    We caution however that while this method is a useful approximation, the resulting
+    integrated flux measurements can be off by a factor of ~2. Furthermore, the central
+    pixel at the core of the galaxy should not be trusted, since this can be off by
+    multiple orders of magnitude.
+