It’s very important to keep a changelog and be disciplined about maintaining it as your product progresses. This applies to both product management and software development/management processes.
I want to mention a great source that talks, describe and almost accepted as a standard in open source community for changelogs:
https://keepachangelog.com
What is a changelog?
A changelog is a file that contains a curated, chronologically ordered list of notable changes for each version of a project.
Why keep a changelog?
To make it easier for users and contributors to see precisely what notable changes have been made between each release (or version) of the project.
Who needs a changelog?
People do. Whether consumers or developers, the end-users of software are human beings who care about what’s in the software. When the software changes, people want to know why and how.
An example Changelog (from Stretchly)
https://gist.github.com/mfyz/da277a38ba7d11bf1c6258a63d12dce6.js
Github renders this markdown changelog beautifully: https://github.com/hovancik/stretchly/blob/master/CHANGELOG.md