Updated CHANGELOG.md

Author: tinywombat765

CHANGELOG.md

@@ -16,6 +16,8 @@ This project adheres to [Semantic Versioning](http://semver.org/).
   ([#448](https://github.com/nix-rust/nix/pull/448))
 - Added `getpgid` in `::nix::unistd`
   ([#433](https://github.com/nix-rust/nix/pull/433))
+- Added `tcgetpgrp` and `tcsetpgrp` in `::nix::unistd`
+  ([#451](https://github.com/nix-rust/nix/pull/451))
 
 ### Changed
 - The minimum supported version of rustc is now 1.7.0.