Add license information to code repository

GitHub's recommended community standards say we should have a license in our repository: https://github.com/otwcode/otwarchive/community

We already specify our license in the site footer (GNU General Public License, version 2: http://www.gnu.org/licenses/gpl-2.0.html), so adding it to the repository won't be a problem.

To test

Make sure a file named LICENSE exists on https://github.com/otwcode/otwarchive and it is indeed GNU GPL version 2.