add django-geojson template filter to the documentation

[lukasalexanderweber]

Thanks for both django-geojson and django-leaflet, it's really convenient to use!

In django-geojson is stated that I can simply put

// Leaflet JS
L.geoJson({{ object_list|geojsonfeature|safe}}).addTo(map);

to add data from my queryset to a map. However, it seems to be a bit more complicated.

I thought it might be a good idea to contribute a working solution. If you think it is correct, we might also want to update the django-geojson docs or link to this page.

[Gagaro]

Thanks for the PR 👍 . Feel free to do another one in django-geojson.