Added missing link and info
Added missing link and info

Added a link to Marktplatz when it's mentioned and also added an info that the support category on Marktplatz is only accessible when signed in. The latter should prevent user frustration and reportings of a broken link in the support category on Marktplatz.

DemokratieInBewegung/abstimmungstool,DemokratieInBewegung/abstimmungstool,DemokratieInBewegung/abstimmungstool,DemokratieInBewegung/abstimmungstool