Add CHANGELOG entry

Author: Gleb Pomykalov

CHANGELOG.md

@@ -17,6 +17,8 @@ This project adheres to [Semantic Versioning](http://semver.org/).
   identity for filesystem checks per-thread.
   (#[1163](https://github.com/nix-rust/nix/pull/1163))
 - Derived `Ord`, `PartialOrd` for `unistd::Pid` (#[1189](https://github.com/nix-rust/nix/pull/1189))
+- Added support for `sendmmsg` and `recvmmsg` calls on Linux and FreeBSD
+  (#[1208](https://github.com/nix-rust/nix/pull/1208))
 
 ### Changed
 - Changed `fallocate` return type from `c_int` to `()` (#[1201](https://github.com/nix-rust/nix/pull/1201))