Add documentation for JSObserver

Adds documentation for JSObserver and updates documentation for the
observer option in general

docs/config.rst

@@ -165,6 +165,8 @@ the form ``key = value``. Two items take a different form:, ``worlds`` and
 
     **You must specify at least one render**
 
+.. _outputdir:
+
 ``outputdir = "<output directory path>"``
     This is the path to the output directory where the rendered tiles will
     be saved.
@@ -199,11 +201,38 @@ the form ``key = value``. Two items take a different form:, ``worlds`` and
     observer to be used. The observer object is expected to have at least ``start``,
     ``add``, ``update``, and ``finish`` methods.
 
-    e.g.::
+    If you want to specify an observer manually, try something like:
+    ::
 
+        from observer import ProgressBarObserver()
         observer = ProgressBarObserver()
 
-.. _outputdir:
+    There are currently three observers available: ``LoggingObserver``, 
+    ``ProgressBarObserver`` and ``JSObserver``. 
+
+    ``LoggingObserver``
+         This gives the normal/older style output and is the default when output
+         is redirected to a file or when running on Windows
+
+    ``ProgressBarObserver``
+        This is used by default when the output is a terminal. Displays a text based
+        progress bar and some statistics.
+
+    ``JSObserver``
+        This will display render progress on the output map in the bottom right
+        corner of the screen. ``JSObserver`` must be invoked with two parameters.
+
+        The first is output directory. For simplicity, specify this as ``outputdir``
+        and place this line after setting ``outputdir = "<output directory path>"``.
+        
+        The second parameter is the  minimum interval between progress updates in 
+        seconds. Progress information won't be written to file or requested by 
+        your web browser more frequently than this interval. 
+        ::
+
+                from observer import JSObserver
+                observer = JSObserver(outputdir, 10)
+
 
 
 .. _renderdict: