Updating LICENSE.md to include required header.

LICENSE.md

+com.unity.perception copyright © 2020 Unity Technologies ApS
+
                                  Apache License
                            Version 2.0, January 2004
                         http://www.apache.org/licenses/

com.unity.perception/LICENSE.md

+com.unity.perception copyright © 2020 Unity Technologies ApS
+
                                  Apache License
                            Version 2.0, January 2004
                         http://www.apache.org/licenses/