README: Add the licence.

README.md

@@ -55,3 +55,16 @@ When submitting changes, try to make them as similar as possible in
 style to the existing code, which approximately follows the style used
 across the Racket code base. If you use Emacs with Racket Mode or
 DrRacket, you should be fine with default settings.
+
+## License
+
+Like the generic graph library, this library is licensed under the Apache
+License, Version 2.0 (the "License"); you may not use this file except in
+compliance with the License.  You may obtain a copy of the License at
+
+    @hyperlink["http://www.apache.org/licenses/LICENSE-2.0"]{http://www.apache.org/licenses/LICENSE-2.0}
+
+Unless required by applicable law or agreed to in writing, software distributed
+under the License is distributed on an @bold{"as is" basis, without warranties
+or conditions of any kind}, either express or implied.  See the License for the
+specific language governing permissions and limitations under the License.