Document automatic mailing lists in the admin UI

Ghost User
2017-12-10T21:12:33+01:00

Add a few lines of text to the admin interface that describe what the automatic mailing lists do.

Spin-off of #388

### Why?

Nobody reads code, nobody properly communicates this information to the next boards.

### Current implementation

People know(tm)

### Desired implementation

Include some text in the admin UI describing `leden@` etc.