update readme

Author: TiteiKo

README.md

@@ -39,6 +39,9 @@ ApiPagination.configure do |config|
 
   # By default, this is set to 'Per-Page'
   config.per_page_header = 'X-Per-Page'
+
+  # Optional : set this to add a header with the current page number.
+  config.page_header = 'X-Page'
 end
 ```