Warn that the Yahoo BOSS Placefinder API is being discontinued.

https://developer.yahoo.com/boss/placefinder/

the Yahoo lookup will be fully removed via:
https://github.com/alexreisner/geocoder/pull/985

README.md

@@ -435,6 +435,7 @@ The [Google Places Details API](https://developers.google.com/places/documentati
 * **Limitations**: "If your application displays Places API data on a page or view that does not also display a Google Map, you must show a "Powered by Google" logo with that data."
 
 #### Yahoo BOSS (`:yahoo`)
+**Warning - [this API will be discontinued on March 31.](https://developer.yahoo.com/boss/placefinder/)**
 
 * **API key**: requires OAuth consumer key and secret (set `Geocoder.configure(:api_key => [key, secret])`)
 * **Key signup**: http://developer.yahoo.com/boss/geo/

lib/geocoder/configuration.rb

@@ -17,6 +17,10 @@ module Geocoder
     if !options.nil?
       Configuration.instance.configure(options)
     end
+
+    if config.lookup == :yahoo
+      Geocoder.log(:warn, "Yahoo BOSS Placefinder API will be discontinued March 31, 2016.")
+    end
   end
 
   ##

lib/geocoder/lookups/yahoo.rb

@@ -28,6 +28,7 @@ module Geocoder::Lookup
     private # ---------------------------------------------------------------
 
     def results(query)
+      Geocoder.log(:warn, "Yahoo BOSS Placefinder API will be discontinued March 31, 2016.")
       return [] unless doc = fetch_data(query)
       doc = doc['bossresponse']
       if doc['responsecode'].to_i == 200