Nit: mention autogen.sh

git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@1472 d4fa192b-c00b-0410-8231-f00ffab90ce4

INSTALL

@@ -4,6 +4,9 @@ Varnish uses the GNU autotools.  To build and install Varnish, simply
 run the 'configure' script in the top-level directory, then run 'make'
 and 'make install'.
 
+If you obtained the sources directly from the Subversion repository,
+you will need to run autogen.sh first to create the 'configure' script.
+
 Additional 'configure' options of interest:
 
   --enable-developer-warnings