chore: add CHANGELOG file for app

[mirdono]

In other apps there is a habit to keep an explicit changelog which, among other things, documents the steps to follow to deploy a release. In the past I found this rather handy, especially if multiple people contributed to a single release (and/or too much time has passed since deploying to DEV for my memory to recall what I did at the time.)

I would propose we also do this for this app. Although, it will probably be less crucial here due to the POC-nature of the app, with possibly few (if any) releases. The idea would be that

• As part of each PR one updates the changelog accordingly. More specifically, add the appropriate entries to the "Unreleased" section.
• When doing a release one moves the contents of the "Unreleased" to a release section, typically titled like "vX.Y.Z (release date)". Afterwards, commit and tag accordingly.

In this PR I added a small example of what an entry would be for the functionality added in another PR. Some examples of more extensive changelogs can be, for example, found in Digitaal Loket or OrganisatiePortaal.