Document that we (will) require python-daemon

We want a daemon running to handle features which require polling, like
monitoring the alarm status and logging temperature data.
First version of the domd daemon

It will be responsible for handing all features which require polling
the PLC and thus can't be implemented in the webapp itself.

Initially planned:
 - Alarm monitoring and notification (e-mail / SMS)
 - Temperature logging (RRDTool?)