Add https://github.com/stoefln to sponsors listing

Author: s1hofmann

Diff for: README.md

@@ -42,6 +42,7 @@ A huge **"Thank you!"** goes out to all sponsors who make open source a bit more
 
 [<img src="https://avatars.githubusercontent.com/u/17616211?v=4" width="75" alt="Reiss Cashmore" />](https://github.com/Reiss-Cashmore)
 [<img src="https://avatars.githubusercontent.com/u/1794527?v=4" width="75" alt="Chet Corcos" />](https://github.com/ccorcos)
+[<img src="https://avatars.githubusercontent.com/u/562800?v=4" width="75" alt="Stephan Petzl" />](https://github.com/stoefln)
 
 # Demo