Add news bullet for #400

Author: hadley

NEWS.md

@@ -8,6 +8,11 @@
 
 * `print()` is faster for very long `xml_nodeset` inputs (#366, @michaelchirico).
 
+* `xml_attr()`, `xml_attrs()`, `xml_double()`, `xml_integer()`, `xml_length()`,
+  `xml_name()`, `xml_path()`, `xml_text()`, and `xml_type()` no longer use
+  S3 dispatch but instead dispatch in C, leading to considerable performance
+  improvements in many cases (@mgirlich, #400).
+
 * `xml_find_int()` analogous to `xml_find_num()` for returning integers 
   matched by an XPath (#365, @michaelchirico).