Prepare README and CHANGELOG for release

[mig42]

Add a section in README.md to describe what to do to migrate from 3.x to 4.x and a section in CHANGELOG.md to write the release notes for 4.0

• [X] Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• [X] Ensure that the pull request title represents the desired changelog entry
• [X] Please describe what you did
• [X] Link to relevant issues in GitHub or Jira
• [X] Link to relevant pull requests, esp. upstream and downstream changes
• [X] Ensure you have provided tests - that demonstrates feature works or fixes the issue