Add a release checklist step to highlight changes

In an effort to better communicate with our users that they might need
to take certain actions when upgrading, we should extract from the
changelog the items that require some action (example: the new version
automatically generates keys, if you want them to be offline - make sure
to create them before upgrading).

doc/HACKING/ReleasingTor.md

@@ -98,7 +98,12 @@ new Tor release:
    to a release-0.2.x branch, manually commit the changelogs to the later
    git branches too.
 
-3. If you're doing the first stable release in a series, you need to
+3. If there are changes that require or suggest operator intervention
+   before or during the update, mail operators (either dirauth or relays
+   list) with a headline that indicates that an action is required or
+   appreciated.
+
+4. If you're doing the first stable release in a series, you need to
    create a ReleaseNotes for the series as a whole.  To get started
    there, copy all of the Changelog entries from the series into a new
    file, and run `./scripts/maint/sortChanges.py` on it.  That will