Add forward/reverse geocoding attributes warning.

README.rdoc

@@ -113,6 +113,33 @@ and make sure it has +latitude+ and +longitude+ attributes, as well as an +addre
   reverse_geocoded_by :lat, :lon, :address => :location
 
 
+== Forward and Reverse Geocoding in the Same Model
+
+If you apply both forward and reverse geocoding functionality to the same model, you can provide different methods for storing the fetched address (reverse geocoding) and providing an address to use when fetching coordinates (forward geocoding), for example:
+
+  class Venue
+
+    # build an address from street, city, and state attributes
+    geocoded_by :address_from_components
+
+    # store the Google-provided address in the full_address attribute
+    reverse_geocoded_by :latitude, :longitude, :address => :full_address
+  end
+
+However, there can be only one set of latitude/longitude attributes, and whichever you specify last will be used. For example:
+
+  class Venue
+
+    geocoded_by :address,
+      :latitude  => :fetched_latitude,  # this will be overridden by the below
+      :longitude => :fetched_longitude  # same here
+
+    reverse_geocoded_by :latitude, :longitude
+  end
+
+The reason for this is that we don't want ambiguity when doing distance calculations. We need a single, authoritative source for coordinates!
+
+
 == SQLite
 
 SQLite's lack of trigonometric functions requires an alternate implementation of the +near+ method (scope). When using SQLite, Geocoder will automatically use a less accurate algorithm for finding objects near a given point. Results of this algorithm should not be trusted too much as it will return objects that are outside the given radius.