Add changelog entry for nix-rust#733

Author: JayH5

CHANGELOG.md

@@ -38,6 +38,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
   ([#602](https://github.com/nix-rust/nix/pull/774))
 - Added `ptrace::Options::PTRACE_O_EXITKILL` on Linux and Android.
   ([#771](https://github.com/nix-rust/nix/pull/771))
+- 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))