update changelog

Add the remarkable changes since v0.1.0 to the
changelog as the last step before v0.2.0.

Author: Andreas Auernhammer

CHANGELOG.md

@@ -4,6 +4,22 @@ 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).
 
+## [0.2.0] - 2019-10-13
+### Added
+ - An (experimental) `sioutil` package with a similar scope like the std library `io/ioutil`
+   package.
+ - Fuzzing using [go-fuzz](https://github.com/dvyukov/go-fuzz) and [fuzzbuzz.io][https://fuzzbuzz.io].
+       
+### Changed
+ - All exported errors are now actual constants instead of effectively constant variables. 
+   ([27e1114](https://github.com/secure-io/sio-go/commit/27e11147b5ddc0a6cbc69d4f79c2273a70ce36eb))
+   Also the `ErrAuth` has been renamed to `NotAuthentic`.
+   This is a major breaking change since:
+     1. An exported symbol has changed.
+     2. The type of exported symbols has changed.
+ - The [go documentation](https://godoc.org/github.com/secure-io/sio-go) has changed to
+   be more explicit about how to use the API and what guarantees are provided.
+
 ## [0.1.0] - 2019-05-24
 ### Added
  - A first stream-based secure channel implementation for wrapping [`io.Reader`](https://golang.org/pkg/io/#Reader)