chore: add COPYRIGHT file and update license references

- move copyright information from LICENSE-* files to COPYRIGHT
- use some improved wording used by the Rust project

COPYRIGHT

@@ -0,0 +1,8 @@
+Copyright (c) Michael Howell
+Copyright (c) uutils developers
+
+Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+<LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+option. All files in the project carrying such notice may not be
+copied, modified, or distributed except according to those terms.

LICENSE-APACHE

@@ -1,6 +1,3 @@
-Copyright (c) Michael Howell
-Copyright (c) uutils developers
-
                               Apache License
                         Version 2.0, January 2004
                      http://www.apache.org/licenses/

LICENSE-MIT

@@ -1,6 +1,3 @@
-Copyright (c) Michael Howell
-Copyright (c) uutils developers
-
 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

README.md

@@ -54,4 +54,8 @@ $ cargo run -- -u fruits_old.txt fruits_new.txt
 
 ## License
 
-diffutils is licensed under the MIT and Apache Licenses - see the `LICENSE-MIT` or `LICENSE-APACHE` files for details
+This project is distributed under the terms of both the MIT license and the
+Apache License (Version 2.0).
+
+See [LICENSE-APACHE](LICENSE-APACHE), [LICENSE-MIT](LICENSE-MIT), and
+[COPYRIGHT](COPYRIGHT) for details.