relvalconsumer: explain why 'relval' package is installed

...because I'd totally forgotten and was thinking it maybe could
be removed.

Signed-off-by: Adam Williamson <awilliam@redhat.com>

roles/relvalconsumer/tasks/main.yml

@@ -24,6 +24,8 @@
 #  - packages
 - name: Install required packages
   dnf:
+    # 'relval' itself is needed as we call it directly for size
+    # checking
     name: ['python3-fedfind', 'python3-wikitcms', 'python3-fedmsg',
            'python3-mwclient', 'python3-setuptools', 'relval']
     state: present