Add market data for Stuttgart

[hbruch]

Data source was Stuttgarter Wochenmärkte. However, as coordinates were mostly not included in the source and the displayed coordinates sometimes a bit off, I used OSM to retrieve them.

To cite OSM as data_source, I added a new data_source_coords. To refer to the underlying OSM features, I added an osm_id, where a corresponding feature exists. Please check if you are ok with this.

[johnjohndoe]

Hi @hbruch. Can you please check your coordinates - here is a preview - you can find the updated link at the end of your pull request.

[johnjohndoe]

@hbruch Thanks a lot for adding Stuttgart 🍷

Please note that I squashed your commits into one. In case you want to keep the OpenStreetMap information I pushed the temporary stuttgart-openstreetmap branch. Please take the commits to your fork and let me know when I can remove it here.

As usual here are the follow-up tasks to promote the project/website:

• Send a tweet #woistmarkt with a screenshot and a link to the website (please share the tweet link here)
• Try to convince the city of Stuttgart to link to our website, see Promote website #38