Add GitHub donations

Author: rnestler

slides/contributing.tex

@@ -14,7 +14,7 @@ \section{Contributing}
       \item Participate in issues
       \item Open pull requests
       \item Improve documentation
-      \item Donate (Patreon or Bitcoin)
+      \item Donate (Patreon, Bitcoin or GitHub donations)
     \end{itemize}
   }
-\end{frame}
+\end{frame}