Add slate to the format list

[skip ci]

Author: oestrich

README.md

@@ -89,7 +89,7 @@ RspecApiDocumentation.configure do |config|
 
   # An array of output format(s).
   # Possible values are :json, :html, :combined_text, :combined_json,
-  #   :json_iodocs, :textile, :markdown, :append_json
+  #   :json_iodocs, :textile, :markdown, :append_json, :slate
   config.format = [:html]
 
   # Location of templates