It's located in the main directory of the game. However, the progressive changelog isn't complete (or that detailed). It only mentions the changes made after MabusAltarn took over the project.
The only way to learn the previous changes, is to download the zip-file for the Community Update v6.0.2 from the archive and take a look at all of the changelogs for the individual files. There is a link to the archive in the CU thread.
Edit: Before I forget, there are a couple more changes incoming. It's mostly fixing some typos, but also a few major changes. Primarily changing farms back to provide food production, because the percentage bonus caused issues for the AI, and re-adding some planetary improvements that got removed. Here is the full changelog.
I sent the necessary files in to Stardock yesterday, and haven't got a response yet. So, no idea when the update comes out.