Re-add "Gogs authors" copyright (togheter with "Gitea authors")

LICENSE

@@ -1,4 +1,5 @@
-Copyright (c) The Gitea Authors
+Copyright (c) 2016 The Gitea Authors
+Copyright (c) 2015 The Gitea Authors
 
 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal