adding current year and copyright holder in LICENSE.md.(Fix-#3738) (#3770)

Author: ac-optimus

LICENSE

@@ -197,7 +197,7 @@ All rights reserved.
       same "printed page" as the copyright notice for easier
       identification within third-party archives.
 
-   Copyright [yyyy] [name of copyright owner]
+   Copyright 2020 The PyMC Developers
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.