Include our copyright statement in license.txt

This is merely making the license file correct. This doesn't change our copyright or licensing whatsoever.

Author: Chris Sanders

LICENSE.txt

@@ -1,5 +1,8 @@
 The MIT License (MIT)
 
+Copyright (c) SpongePowered <https://www.spongepowered.org>
+Copyright (c) contributors
+
 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal
 in the Software without restriction, including without limitation the rights