feat: add ludwigsburg

[dersvenhesse]

Added Ludwigsburg as new city and unified minor wordings in readme.

[dersvenhesse]

@johnjohndoe, done.

[johnjohndoe]

@dersvenhesse Now that Ludwigsburg is on the map it is time for you to advertise them. There is a couple of ways to do this - and please let me know about your choice - but here is a few ideas:

• [ ] Tweet it out - don't forget a nice screenshot :bird: Or post it on Facebook
• [ ] Ask the city of Ludwigsburg if they put a link and maybe a screenshot on their website. Some cities did that and one can clearly see the user increase in the stats! :cityscape:
• [ ] Share the news with local newspapers :newspaper:
• [ ] Print stickers and share them: the assets can be found in the wo-ist-markt-media repository :printer:
• [ ] Become a living advertisement - print your own t-shirt - I did that! Same assets as above :shirt:

Also check issue #38.