Add changelog (#17)

Author: gahjelle

CHANGELOG.md

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+# Changelog
+
+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).
+
+
+## [Unreleased](compare/v1.0.0...HEAD)
+
+### Added
+
+- `.last` attribute with the value of the last measured time (by [@janfreyberg](https://github.com/janfreyberg) in [#13](pull/13)).
+- `CHANGELOG.md` detailing changes made to `codetiming` since its initial publication.
+
+
+## [1.0.0](releases/tag/v1.0.0) - 2019-12-31
+
+Initial version of `codetiming`. Version 1.0.0 corresponds to the code in the tutorial [Python Timer Functions: Three Ways to Monitor Your Code](https://realpython.com/python-timer/).