[4.0] 493b979 Document the version marker

[Per Buer]

commit 493b979fc5b01ce2203ad91f85a107e22281bcda
Author: Per Buer <perbu at varnish-software.com>
Date:   Wed Mar 19 11:28:09 2014 +0100

    Document the version marker

diff --git a/doc/sphinx/reference/vcl.rst b/doc/sphinx/reference/vcl.rst
index 6f91b91..43295bb 100644
--- a/doc/sphinx/reference/vcl.rst
+++ b/doc/sphinx/reference/vcl.rst
@@ -40,6 +40,10 @@ In addition VCL has the following constructs:
 
 Note that are no loops or iterators of any kind in VCL.
 
+Each VCL file must start by declaring its version. For Varnish 4.0 you
+must include a "vcl 4.0;" at the top of the file.
+
+
 Operators
 ---------