Create a pull request template

[sharkwouter]

Contributors should be asked to:

• [ ] Update CHANGELOG.md when their change will apply for people using the latest release
• [ ] Add themselves to the credits in about.ui
• [ ] Add themselves to special thanks in the README.md
• [ ] Make sure the linter gives no errors
• [ ] Make sure all tests run
• [ ] Implement unit tests is applicable
• [ ] Add their translation to SUPPORTED_LOCALES in constants.py if their work is a translation
• [ ] Talk about new features on Discord or in an issue

Not all of this should be mandatory, but it would be nice to point users in the right direction, because I'm getting lost in what changed in between releases 😅