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:
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)
Github renders this markdown changelog beautifully: https://github.com/hovancik/stretchly/blob/master/CHANGELOG.md