Add Geocoder.geographic_center to documentation.

README.rdoc

@@ -73,6 +73,9 @@ Some utility methods are also available:
   
   # look up coordinates of some location (like searching Google Maps)
   Geocoder.fetch_coordinates("25 Main St, Cooperstown, NY")
+  
+  # find the geographic center (aka center of gravity) of several points
+  Geocoder.geographic_center([ [40.22,-73.99], [40.72,-73.98], [40.57,-74.61] ])
 
 
 == More On Configuration