update changelog

Add the remarkable changes since v0.2.0 to the
changelog as the last step before v0.3.0.

Author: Andreas Auernhammer

CHANGELOG.md

@@ -4,6 +4,19 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [0.3.9] - 2019-10-29
+### Added
+ - The [`Algorithm`](https://godoc.org/github.com/secure-io/sio-go#Algorithm) type and
+   four commonly used AEAD algorithms (AES-128-GCM, AES-256-GCM, ChaCha20-Poly1305 and
+   XChaCha20-Poly1305).  
+   The [`Algorithm.Stream`](https://godoc.org/github.com/secure-io/sio-go#Algorithm.Stream)
+   method provides a way to create a `Stream` directly from a secret key instead of first 
+   creating an AEAD instance explicitly.
+ - The [`NativeAES`](https://godoc.org/github.com/secure-io/sio-go/sioutil#NativeAES) function
+   in the `sioutil`. It allows users to determine whether an optimized (and constant time)
+   implementation is available for their CPU.
+ - Two helper functions (`Random` and `MustRandom`) for generating random bytes in `sioutil`.
+
 ## [0.2.0] - 2019-10-13
 ### Added
  - An (experimental) `sioutil` package with a similar scope like the std library `io/ioutil`