[Dev] [PATCH] README.maintenance: clarify how to become redmine admin

[bill-auger]

i suppose that as this is being discussed on the mailing list
now, i should add for clarification, the reason why any of that
is there

the simplest thing to do would be to make every team member a
web admin permanently - it is mainly just a security concern;
because the redmine admins controls are very brittle and
difficult to understand - it is a very rare occasion when
modifications to the redmine config are required; so its just
best that no one has permanent admin access via the web