GitHub uses a library called Licensee to identify a project's license
type. It shows this information in the status bar and via the API if it
can unambiguously identify the license.

This commit modifies a few of MariaDB's docs so that Licensee is able
to recognize the repository's license type. It renames the README's
"License" section to "Licensing" and renames COPYING.thirdparty to
THIRDPARTY.

These changes allow Licensee to bypass both files when it
scans the repo for license files, which thus allows Licensee to
successfully identify the license type of MariaDB as GPL 2.0.
Signed-off-by: Andrea Kao <eirinikos@gmail.com>