Update changelog for PR#739

Author: Susurrus

CHANGELOG.md

@@ -16,6 +16,10 @@ This project adheres to [Semantic Versioning](http://semver.org/).
   ([#647](https://github.com/nix-rust/nix/pull/647))
 - Added the `pid()` method to `WaitStatus` for extracting the PID.
   ([#722](https://github.com/nix-rust/nix/pull/722))
+- Expose `uname()` on all platforms.
+  ([#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))
 
 ### Changed
 - Renamed existing `ptrace` wrappers to encourage namespacing ([#692](https://github.com/nix-rust/nix/pull/692))