docs: add changelog

Author: matejcik

CHANGELOG.rst

@@ -0,0 +1,45 @@
+Changelog
+=========
+
+.. default-role:: code
+
+All notable changes to this project will be documented in this file.
+
+The format is based on `Keep a Changelog`_, and this project adheres to
+`Semantic Versioning`_.
+
+`0.20`_ - 2021-07-27
+---------------------
+
+.. _0.20: https://github.com/trezor/python-mnemonic/compare/v0.19...v0.20
+
+Added
+~~~~~
+
+- Type annotations
+- Support for testnet private keys
+
+Changed
+~~~~~~~
+
+- Project directory structure was cleaned up
+- Language on the `Mnemonic` object is remembered instead of repeatedly detecting
+
+Removed
+~~~~~~~
+
+- Support for Python 2.7 and 3.4 was dropped
+
+
+
+0.19 - 2019-10-01
+------------------
+
+Added
+~~~~~
+
+- Start of changelog
+
+
+.. _Keep a Changelog: https://keepachangelog.com/en/1.0.0/
+.. _Semantic Versioning: https://semver.org/spec/v2.0.0.html