Added help text to explain that an index is required to take advantage of the...

Added help text to explain that an index is required to take advantage of the bounding box optimisation

README.md

@@ -527,6 +527,15 @@ Calling `obj.coordinates` directly returns the internal representation of the co
 
 For consistency with the rest of Geocoder, always use the `to_coordinates` method instead.
 
+Optimisation of Distance Queries
+--------------------------------
+
+In MySQL and Postgres the finding of objects near a given point is speeded up by using a bounding box to limit the number of points over which a full distance calculation needs to be done.
+
+To take advantage of this optimisation you need to add a composite index on latitude and longitude. In your Rails migration:
+
+    add_index :table, [:latitude, :longitude]
+
 
 Distance Queries in SQLite
 --------------------------