Add text of GPLv3 to repo; move LICENSE file into README

bgilbert commented 2 months ago

The LICENSE file says:

You should have received a copy of the GNU General Public License
along with this program.  If not, see <https://www.gnu.org/licenses/>.

Add a copy of the GPLv3 under the conventional name COPYING.

In addition, move the contents of the LICENSE file into the License section of README.md and clean it up a bit; it's probably good to make the license statement and warranty disclaimer more prominent. Also, if we kept LICENSE as a separate file, the GitHub web UI would think there's a second license it doesn't understand; see screenshots below.