Update changelog [ci skip]

Author: n-rodriguez

CHANGELOG.md

@@ -4,6 +4,7 @@
 
 * Add rudimentary support for Microsoft SQL Server
 * Fixes errors when options[param] is nil [PR 315](https://github.com/jbox-web/ajax-datatables-rails/pull/315) (thanks @allard)
+* Improve query performance when nulls_last option is enabled [PR 317](https://github.com/jbox-web/ajax-datatables-rails/pull/317) (thanks @natebird)
 * Add :string_in cond [PR 323](https://github.com/jbox-web/ajax-datatables-rails/pull/323) (thanks @donnguyen)
 * Update documentation
 * Test with latest Rails and Ruby versions