Add donation link to flatpak manifest

Author: rolandgeider

flatpak/de.wger.flutter.metainfo.xml

@@ -14,6 +14,7 @@
     <developer_name>wger</developer_name>
     <url type="homepage">https://wger.de/</url>
     <url type="bugtracker">https://github.com/wger-project/flutter/issues</url>
+    <url type="donation">https://buymeacoffee.com/wger</url>
 
     <description>
         <p>