add contribution hints and polish the README a bit

[thombergs]

• [ 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