Document proftpd configuration

Change-Id: I354a8168a5ebc3ef204c6bf677359b7c05394715
Merge remote-tracking branch 'gerrit/5696_see_proftpd_logs_in_project_admin' into stable

* gerrit/5696_see_proftpd_logs_in_project_admin:
  Document proftpd configuration

Change-Id: I3158351eed5ce740e17a9883f344d9fd04cc38bf
This is Tuleap 6.9.99.15

Change-Id: Ie324a06c84ac125291cce0f1d5cafb94084e5c68