Clarified contributing guidelines

README.md

@@ -119,8 +119,10 @@ To generate a binary package, execute `make package`
 
 ## Contributing
 
-Contributions are always welcome. You can submit them as pull requests or via
-email to `tastytea`@`tastytea.de`.
+Contributions are always welcome. You can submit them as pull requests on
+schlomp.space or via email to `tastytea`@`tastytea.de` (ideally using
+`git format-patch` or `git send-email`). Please do not submit them via GitHub,
+it is just a mirror.
 
 ## Contact