branching checklist: More detailed instructions re MAINTAINERS

Provide a rune, following which a magit selective git add
(or git add -p) can be used to commit the appropriate changes.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>

docs/process/branching-checklist.txt

@@ -88,6 +88,8 @@ Update new stable tree's MAINTAINERS to contain correct info for this
 stable branch: usually, copy text from previous
 staging-$ov:MAINTAINERS section "Stable Release Maintenance"
 into new staging-$v, deleting what's there.
+  git cat-file blob origin/staging-$ov:MAINTAINERS >MAINTAINERS
+and review the changes, commiting ONLY THE RELEVANT ONES
 
 Set off a manual osstest run, since the osstest cr-for-branches change
 will take a while to take effect: