Add credit and link to SimianGrid project. Thanks, guys!

OpenSim/Services/MapImageService/MapImageService.cs

@@ -23,6 +23,10 @@
  * 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.
+ * 
+ * The design of this map service is based on SimianGrid's PHP-based 
+ * map service. See this URL for the original PHP version:
+ * https://github.com/openmetaversefoundation/simiangrid/
  */
 
 using System;