[docs] Add changelog note about move based set_body references #792

Author: Peter Thorson

changelog.md

@@ -22,6 +22,8 @@ HEAD
 - Performance: Refactor to_hex utility method to reduce unecessary copying,
   std::string construction, and duplicated code. Add tests. Thank you Dmitry
   Matrokhin for reporting and the patch. #914
+- Performance: Move based overload for connection::set_body for C++11 and
+  later compilers. Thank you Matus Kysel for the patch & tests. #792
 - Reliability: Add a few defensive assertions to guard against corrupted
   HTTP message reads. Thank you Oleh Derevenko for reporting. #899
 - Documentation: Added language to explicitly clarify that the library