Add Changelog feature for addon devs to display what is new in an addon update

**Is your feature request related to a problem? Please describe.**
Not necessarily. It would be nice as an end user to know what was updated in the latest addon that needs to be updated.

**Describe the solution you'd like**
Perhaps a dialog box or some sort of output when installing an update of a specific addon to output what has changed in this version update.

**Describe alternatives you've considered**
Providing a link back to the repository somehow?

**Additional context**
Several addon devs provide changelogs in their READMEs. For example:
* https://github.com/mwganson/Editor_Assistant#changelog
* https://github.com/Zolko-123/FreeCAD_Assembly4#release-notes
* https://github.com/shaise/FreeCAD_SheetMetal#release-notes
* https://github.com/shaise/FreeCAD_FastenersWB#release-notes-berberic2