Update README

I have added additional information to the README. The user needs to
configure the CKAN configuration file as well. Also added a note about
the ckanext-harvest dependency.

Signed-off-by: Konrad Reiche <konrad.reiche@fokus.fraunhofer.de>
Merge remote-tracking branch 'origin/master'

Conflicts:
	ckanext/govdatade/harvesters/ckanharvester.py

Signed-off-by: Konrad Reiche <konrad.reiche@fokus.fraunhofer.de>