[master] 1bfed1784 Add a brief intro to 'Upgrading to 6.1'.

[Geoff Simmons]

commit 1bfed17848892445faff8338fff1eb59a3eb5ad4
Author: Geoff Simmons <geoff at uplex.de>
Date:   Sun Sep 16 19:19:01 2018 +0200

    Add a brief intro to 'Upgrading to 6.1'.

diff --git a/doc/sphinx/whats-new/upgrading-6.1.rst b/doc/sphinx/whats-new/upgrading-6.1.rst
index 131fd73a8..94723e600 100644
--- a/doc/sphinx/whats-new/upgrading-6.1.rst
+++ b/doc/sphinx/whats-new/upgrading-6.1.rst
@@ -4,6 +4,12 @@
 Upgrading to Varnish 6.1
 %%%%%%%%%%%%%%%%%%%%%%%%
 
+A configuration for Varnish 6.0.x will run for version 6.1 without
+changes.  There has been a subtle change in the interpretation of the
+VCL variable ``beresp.keep`` under specific circumstances, as
+discussed below. Other than that, the changes in 6.1 are new features,
+described in the following.
+
 varnishd parameters
 ===================