Update CHANGELOG for PR rust-lang#506

Author: asomers

CHANGELOG.md

@@ -8,6 +8,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 ### Added
 - Added support for POSIX AIO
   ([#483](https://github.com/nix-rust/nix/pull/483))
+  ([#506](https://github.com/nix-rust/nix/pull/506))
 - Added support for XNU system control sockets
   ([#478](https://github.com/nix-rust/nix/pull/478))
 - Added support for `ioctl` calls on BSD platforms