Add a changelog

Seems like a good thing to do early on to track what is being changed in
the firmware. This will file will follow the https://keepachangelog.com/ format
as best as possible.

CHANGELOG.md

@@ -0,0 +1,22 @@
+# 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]
+
+## [0.0.2] - 2021-10-27
+
+### Added
+- This CHANGELOG file to hopefully serve as an evolving example of a
+  standardized open source project CHANGELOG.
+- Fixed the Particle variable declarations for each temperature sensor, as well
+  as "devices" count by moving when they are registered to the `setup` function.
+
+## [0.0.1] - 2021-10-23
+
+### Added
+- Started versioning the firmware, so we can call anything added before this
+  the initial commit.