Clarify when to use a key for Google.

README.rdoc

@@ -264,7 +264,7 @@ The following is a comparison of the supported geocoding APIs. The "Limitations"
 
 ==== Google (<tt>:google</tt>)
 
-API key:: optional (required for Premier)
+API key:: required for Premier (do NOT use a key for the free version)
 Key signup:: http://code.google.com/apis/maps/signup.html
 Quota:: 2,500 requests/day, 100,000 with Google Maps API Premier
 Region:: world