Add changelog entry for #733

Author: JayH5

CHANGELOG.md

@@ -22,6 +22,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
   ([#739](https://github.com/nix-rust/nix/pull/739))
 - Expose `signalfd` module on Android as well.
   ([#739](https://github.com/nix-rust/nix/pull/739))
+- 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))