Add a note if no packages appear in the mail draft

docs/distributions/cc7.md

@@ -95,6 +95,9 @@ Send an email to `linux-announce-test@cern.ch` with updated packages information
 !!! Note ""
     `$EDITOR` must be set to your prefered editor to edit the email
 
+!!! note "No packages in the mail's draft?"
+    If you thought there were some packages to be updated but you don't see any in the draft, it might very well be that these were `debugingo` or `src.rpm`s. As you can see in [this line](https://gitlab.cern.ch/linuxsupport/lxdist-build/blob/master/bin/bsmailusers#L197) we do not send updates for those.
+
 ## Release to production
 
 Generate list of repos that have been updated in the last week