README.md: move COPYING license verbiage back to the README.md file

Now that pkg.go.dev supports the UNLICENSE, the COPYING file confuses
the automatic license detection. Move the license text back to README.md.

This is merely a cosmetic change, there are no licensing changes to
the project.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

COPYING

@@ -1,4 +0,0 @@
-This project is dual-licensed under the UNLICENSE (see UNLICENSE) or
-the MIT license (see LICENSE-MIT).
-
-You may use the project under the terms of either license.

README.md

@@ -19,3 +19,10 @@ Post discussion to the [mailing list](https://lists.sr.ht/~eliasnaur/gio) and pa
 account is required and you can post without being subscribed.
 
 See the [contribution guide](https://gioui.org/doc/contribute) for more details.
+
+## License
+
+This project is dual-licensed under the UNLICENSE (see the UNLICENSE file) or
+the MIT license (see the LICENSE-MIT file).
+
+You may use the project under the terms of either license.