update LICENSE to include GitHub CC-BY license

fixes #11

LICENSE

@@ -25,3 +25,9 @@ DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+----------
+
+Some documentation is taken from the GitHub Developer site
+<http://developer.github.com/>, which is available under a Creative Commons
+Attribution 3.0 License.