Add changelog entry for nix-rust#733

Author: JayH5

CHANGELOG.md

@@ -30,6 +30,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 - Added socket option variant that enables the timestamp socket
   control message: `nix::sys::socket::sockopt::ReceiveTimestamp`
   ([#663](https://github.com/nix-rust/nix/pull/663))
+- Added `nix::unistd::{getgroups, setgroups, getgrouplist, initgroups}`. ([#733](https://github.com/nix-rust/nix/pull/733))
 
 ### Changed
 - Renamed existing `ptrace` wrappers to encourage namespacing ([#692](https://github.com/nix-rust/nix/pull/692))