Bump version to 0.1.1 and add changelog

CHANGELOG.rst

+Changelog
+=========
+
+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`_.
+
+`0.1.1`_ - 2022-03-04
+---------------------
+
+Fixed
+~~~~~
+
+* Add __str__ methods to Product and StampFormat
+
+`0.1`_
+------
+
+Added
+~~~~~
+
+* Initial release.
+
+.. _Keep a Changelog: https://keepachangelog.com/en/1.0.0/
+.. _Semantic Versioning: https://semver.org/spec/v2.0.0.html
+
+.. _0.1: https://edugit.org/hansegucker/porto/-/tags/0.1
+.. _0.1.1: https://edugit.org/hansegucker/porto/-/tags/0.1.1

pyproject.toml

 [tool.poetry]
 name = "porto"
-version = "0.1.0"
+version = "0.1.1"
 description = "Porto provides a unified and simple interface to various online stamp services."
 authors = ["Jonathan Weth <dev@jonathanweth.de>"]
 maintainers = ["Jonathan Weth <dev@jonathanweth.de>"]