edit MariaDB license info so that GitHub recognizes it

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>

CMakeLists.txt

@@ -470,7 +470,7 @@ ADD_CUSTOM_TARGET(INFO_BIN ALL
   WORKING_DIRECTORY ${CMAKE_BINARY_DIR}
 )
 
-INSTALL_DOCUMENTATION(README.md CREDITS COPYING COPYING.thirdparty
+INSTALL_DOCUMENTATION(README.md CREDITS COPYING THIRDPARTY
                       EXCEPTIONS-CLIENT COMPONENT Readme)
 
 # MDEV-6526 these files are not installed anymore

README.md

@@ -39,8 +39,8 @@ https://launchpad.net/~maria-discuss
 and the #maria IRC channel on Freenode.
 
 
-License:
---------
+Licensing:
+----------
 
 ***************************************************************************
 
@@ -51,8 +51,8 @@ General Public License (GPLv2). (I.e. Without the "any later version"
 clause.) This is inherited from MySQL. Please see the README file in
 the MySQL distribution for more information.
 
-License information can be found in the COPYING, COPYING.LESSER,
-and COPYING.thirdparty files.
+License information can be found in the COPYING file. Third party
+license information can be found in the THIRDPARTY file.
 
 ***************************************************************************