Adding changelog for key API and Documentation updates

Author: nlfoster

CHANGELOG.md

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+# Changelog
+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).
+
+## Documentation Update <sub><sup>(February 5, 2021)</sup></sub>
+### Added
+* Updated code examples to use modern practices.
+* `var` has been replaced with `const` or `let`, and all functions have been replaced by arrow functions.
+
+## 1.2.33 <sub><sup>(January 27, 2021)</sup></sub>
+### Added
+* Added `digital-retailing` location for placement of Digital Retailing integration content, such as an iframe.