[MODEL] Fixed incorrect instruction how to extract functionality into a Searchable concern

Due to changing the delegation framework to ActiveSupport::Delegate,
the documented pattern of using class methods defined in `ClassMethods`
stopped working. The documentation has been changed after small research of
alternative solutions.

Thanks to @zzet for outlining the fix.

Closes elastic#20

Author: karmi

elasticsearch-model/README.md

@@ -76,10 +76,8 @@ module Searchable
     mapping do
       # ...
     end
-  end
 
-  module ClassMethods
-    def search(query)
+    def self.search(query)
       # ...
     end
   end