Added explanation regarding unknown coordinates to readme.

README.md

@@ -482,6 +482,30 @@ However, there can be only one set of latitude/longitude attributes, and whichev
 
 The reason for this is that we don't want ambiguity when doing distance calculations. We need a single, authoritative source for coordinates!
 
+It is then possible to call both forward and reverse geocoding sequentially.
+
+For example:
+
+    class Venue
+
+      after_validation :geocode, :reverse_geocode
+
+    end
+
+For certain geolocation services such as Google geolocation API this may cause issues during subsequent updates to database records if the longtitude and latitude attributes are not on a known feature, because the following call:
+
+     after_validation :geocode
+
+will change the longtitude and latitude based on the location field, which would be the closest known location to the original coordinates. In this case it is better to add conditions to each call, as not to override coordinates that do not have features associated with them.
+
+For example:
+
+    class Venue
+
+      after_validation :reverse_geocode, :if => :has_coordinates
+      after_validation :geocode, :if => :has_location, :unless => :has_coordinates
+
+    end  
 
 Use Outside of Rails
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