Add a license file (AGPLv3)

If I'm actually presenting this online (and I am), I should probably
have a real license attached to it. The AGPL seems good to me. I can't
imagine anyone would seriously have conflict concerns for a Boids
implementation.

The webpage now presents this information, as does the Cargo.toml
manifest.

www/index.html

@@ -84,6 +84,10 @@
           <td>Source code</td>
           <td><a href=https://github.com/DerVerrater/another-boids-in-rust>https://github.com/DerVerrater/another-boids-in-rust</a></td>
         </tr>
+        <tr>
+          <td>License</td>
+          <td><a href="https://www.gnu.org/licenses/agpl-3.0.txt">AGPL-3.0-only</a></td>
+        </tr>
         <tr>
           <td>Program Version</td>
           <!-- This version text is completely unchecked. I'll need to do something about that. -->