process/release: mention MAINTAINERS adjustments

For many major releases I've been updating ./MAINTAINERS _after_ the
respective branch was handed over to me. That update, however, is
relevant not only from the .1 minor release onwards, but right from the
.0 release. Hence it ought to be done as one of the last things before
tagging the tree for the new major release.

See the seemingly unrelated parts (as far as the commit subject goes) of
e.g. 9d465658b4 ("update Xen version to 4.20.1-pre") for an example.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Julien Grall <jgrall@amazon.com>

docs/process/release-technician-checklist.txt

@@ -53,6 +53,9 @@ t=RELEASE-$r
     - "Xen 4.5-unstable" on unstable
     - "Xen 4.5-rc" for release candidate
 
+* for major releases, drop staging-only text from MAINTAINERS and add stable
+  maintainer(s) there
+
 * change xen-unstable Config.mk
 #   QEMU_UPSTREAM_REVISION,
 #   QEMU_TRADITIONAL_REVISION