document the compression behaviour in Varnish 3.0

doc/sphinx/tutorial/compression.rst

+.. _tutorial-compression:
+
+Compression
+~~~~~~~~~~~
+
+New in Varnish 3.0 was native support for compression, using gzip
+encoding. *Before* 3.0, Varnish would never compress objects. 
+
+In Varnish 3.0 compression defaults to "on", meaning that it tries to
+be smart and do the sensible thing.
+
+If you don't want Varnish tampering with the encoding you can disable
+compression all together by setting the parameter http_gzip_support to
+*false*. Please see man :ref:`ref-varnishd` for details.
+
+
+Default behaviour
+~~~~~~~~~~~~~~~~~
+
+The default for Varnish is to check if the client supports our
+compression scheme (gzip) and if it does it will override the
+Accept-Encoding header and set it to "gzip".
+
+When Varnish then issues a backend request the Accept-Encoding will
+then only consist of "gzip". If the server responds with gzip'ed
+content it will be stored in memory in its compressed form. If the
+backend sends content in clear text it will be stored like that.
+
+You can make Varnish compress content before storing it in cache in
+vcl_fetch by setting do_gzip to true, like this::
+
+   sub vcl_fetch {
+        if (beresp.http.content-type ~ "text") {
+                set beresp.do_gzip = true;
+        }
+  }
+
+Please make sure that you don't try to compress content that is
+incompressable, like jpgs, gifs and mp3. You'll only waste CPU
+cycles. You can also uncompress objects before storing it in memory by
+setting do_gunzip to *true* but I have no idea why anybody would want
+to do that.
+
+Generally, Varnish doesn't use much CPU so it might make more sense to
+have Varnish spend CPU cycles compressing content than doing it in
+your web- or application servers, which are more likely to be
+CPU-bound.
+
+GZIP and ESI
+~~~~~~~~~~~~
+
+If you are using Edge Side Includes you'll be happy to note that ESI
+and GZIP work together really well. Varnish will magically decompress
+the content to do the ESI-processing, then recompress it for efficient
+storage and delivery. 
+
+
+Clients that don't support gzip
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+If the client does not support gzip the Accept-Encoding header is left
+alone and we'll end up serving whatever we get from the backend
+server. Remember that the Backend might tell Varnish to *Vary* on the
+Accept-Encoding.
+
+If the client does not support gzip but we've already got a compressed
+version of the page in memory Varnish will automatically decompress
+the page while delivering it.
+
+
+A random outburst
+~~~~~~~~~~~~~~~~~
+
+Poul has written :ref:`phk_gzip` which talks abit more about how the
+implementation works. 

doc/sphinx/tutorial/index.rst

@@ -25,6 +25,7 @@ separate topic. Good luck.
 	cookies
 	vary
 	purging
+	compression
 	esi
 	virtualised
 	advanced_backend_servers