Add external_url description to bplan doc
Add external_url description to bplan doc

liqd/adhocracy3.mercator,liqd/adhocracy3.mercator,liqd/adhocracy3.mercator,liqd/adhocracy3.mercator,liqd/adhocracy3.mercator,liqd/adhocracy3.mercator,liqd/adhocracy3.mercator