Create de-nw-city_of_cologne.json

I didn't know, if put german version of city "Köln" in filename as "koeln" or english version "cologne" (used). Same in File, section coverage > city "Köln"?
Create de-he-city_of_frankfurtammain.json
Create de-mv-city_of_rostock.json
Update de-nw-city_of_cologne.json

changed spelling of city in coverage > city to german version of city name
Merge pull request #705 from okilimu/master

add german cities cologne, frankfurt am main and rostock