doc: Update CHANGELOG.md

Author: DanSnow

CHANGELOG.md

@@ -7,6 +7,8 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 ### Added
 - Add IP_RECVIF & IP_RECVDSTADDR. Enable IP_PKTINFO and IP6_PKTINFO on netbsd/openbsd.
   ([#1002](https://github.com/nix-rust/nix/pull/1002))
+- Add killpg
+  ([#1034](https://github.com/nix-rust/nix/pull/1034))
 ### Changed
 - `PollFd` event flags renamed to `PollFlags` ([#1024](https://github.com/nix-rust/nix/pull/1024/))
 - `recvmsg` now returns an Iterator over `ControlMessageOwned` objects rather