Note about newtypes added to changelog.

Author: Kixunil

CHANGELOG.md

@@ -32,6 +32,8 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 - Changed type signature of `sys::select::FdSet::contains` to make `self`
   immutable ([#564](https://github.com/nix-rust/nix/pull/564))
 - Changed type of `sched::sched_setaffinity`'s `pid` argument to `pid_t`
+- Newtypes for UID, GID, and PID. `uid_t` changed to `Uid`, `gid_t` changed to
+  `Gid`, `pid_t` changed to `Pid`.
 
 ### Removed
 - Removed io::Error from nix::Error and conversion from nix::Error to Errno