Add the VRT major bump to changes.rst

This starts a 6.5.1 changes entry, with only the major bump present.

doc/changes.rst

@@ -26,6 +26,14 @@ http://varnish-cache.org/docs/trunk/whats-new/index.html and via
 individual releases. These documents are updated as part of the
 release process.
 
+================================
+Varnish Cache 6.5.1 (2020-09-25)
+================================
+
+* Bump the VRT_MAJOR_VERSION from 11 to 12, to reflect the API changes
+  that went into the 6.5.0 release. This step was forgotten for that
+  release.
+
 ================================
 Varnish Cache 6.5.0 (2020-09-15)
 ================================