Added --post-serve explanation into the nbconvert docs.

docs/source/interactive/nbconvert.rst

@@ -61,8 +61,12 @@ The currently supported export formats are:
 * ``--to slides``
 
   This generates a Reveal.js HTML slideshow.
-  It must be served by an HTTP server.  The easiest way to do this is adding
-  ``--post serve`` on the command-line.
+  It must be served by an HTTP server. The easiest way to do this is adding
+  ``--post serve`` on the command-line. The ``--post serve`` post-processor 
+  proxies Reveal.js requests to a CDN if no local Reveal.js library is present. 
+  For low connectivity environments, just place the Reveal.js library in the 
+  same directory where your_talk.slides.html is located or point to another 
+  directory using the ``--reveal-prefix`` alias.
 
 * ``--to markdown``
 
@@ -224,4 +228,3 @@ The corresponding Python script is::
 
 Note that indeed the output of the code cell, which is present in the JSON 
 container, has been removed in the ``.py`` script.
-