Specify content of description field(and maybe others)

Created by: cburchert

We just had a discussion about what the event description field contains. Our quick result was that markdown would be a good option, as everyone could generate their own formating based on the environment in which the event is rendered. It should be considered, that newsletter entries do not have much space, therefore big amounts of text are not necessary, at the same time it would be desireable to be able to include links and simple text formating. In any case it should be specified in the docs and if possible validated. Another important thing here is XSS. We need to either filter HTML tags or make sure developers know that the api does not prevent the inclusion of HTML tags in fields.