update config docs for observer

docs/config.rst

@@ -188,6 +188,21 @@ the form ``key = value``. Two items take a different form:, ``worlds`` and
 
         processes = 2
 
+.. _observer:
+
+``observer = <observer object>``
+    This lets you configure how the progress of the render is reported. The
+    default is to display a progress bar, unless run on Windows or with stderr
+    redirected to a file. The default value will probably be fine for most
+    people, but advanced users may want to make their own progress reporter (for
+    a web service or something like that) or you may want to force a particular
+    observer to be used. The observer object is expected to have at least ``start``,
+    ``add``, ``update``, and ``finish`` methods.
+
+    e.g.::
+
+        observer = ProgressBarObserver()
+
 .. _outputdir: