Mention other formats in README

Author: oestrich

README.md

@@ -81,7 +81,9 @@ RspecApiDocumentation.configure do |config|
   # Output folder
   config.docs_dir = Rails.root.join("doc", "api")
 
-  # An array of output format(s). Possible values are :json, :html
+  # An array of output format(s).
+  # Possible values are :json, :html, :combined_text, :combined_json,
+  #   :json_iodocs, :textile, :append_json
   config.format = [:html]
 
   # Location of templates