Add missing credits for OSM background image

CREDITS.md

@@ -2,6 +2,8 @@
 
 "Woman Wearing Headphones and Using Cellphone Near a Bus Stop" by Andrea Piacquadio from [Pexels](https://www.pexels.com/photo/woman-wearing-headphones-and-using-cellphone-near-a-bus-stop-5047375/)
 
+OpenStreetMap Screenshot with [Map data from OpenStreetMap](http://openstreetmap.org/copyright)
+
 OpenStreetMap logo by Ken Vermette from [OpenStreetMap](https://wiki.openstreetmap.org/wiki/File:Public-images-osm_logo.svg) based on https://commons.wikimedia.org/wiki/File:OpenStreetMap-Logo-2006.svg licensed under [CC BY-SA 3.0](https://creativecommons.org/licenses/by-sa/3.0/)
 
 "todo list" by Vectors Point from [Noun Project](https://thenounproject.com/icon/todo-list-2577194/) licensed under [CC BY 3.0](https://creativecommons.org/licenses/by/3.0/) / Removed "Created by ..." text
@@ -14,4 +16,4 @@ Apple und das Apple-Logo sind Marken der Apple Inc.
 
 Google Play und das Google Play-Logo sind Marken von Google LLC.
 
-"TU Chemnitz Hauptbahnhof" by TU Chemnitz/Jacob Müller
+"TU Chemnitz Hauptbahnhof" by TU Chemnitz/Jacob Müller