Add PR template

[tmeisel]

Issue

• Solutions Arsenal xx
• Internal
• Chore
• Hotfix

Description

This PR adds a pull request template to .github/PULL_REQUEST_TEMPLATE/pull_request_template.md. This PR follows the suggested template.

Checklist

• PR required dependency updates
• I've added tests that cover the changes
• I've updated the documentation to reflect the new behavior

Additional Authors

none

[trevex]

Solutions Arsenal -> Solution Arsenal?

Maybe just the link is fine as well, as github renders it properly anyway, e.g. #155

[trevex]

@olzemal used the template in #195

The checkboxes have the side-effects that the PR has status 3/7 which confused me, as I thought the PR is not yet ready... We should at least not use checkboxes for multiple choise and should probably include a note to remove checkboxes that are not applicable.

[dermorz]

May I suggest a different approach? I would keep the checkboxes minimal and only keep those that are always applicable, so from the PR list-view we can see a 2/2.

Other than that I would give the contributer prompts to leave relevant information for the reviewer.

Things like linter, coverage, fmt, codegen should be checked automatically and should not have their explicit checkboxes.

For example:

## What
<!-- One sentence summary -->
Closes #

## Why
<!-- Motivation / problem being solved. Skip if obvious from the linked issue. -->

## Testing
<!-- How was this tested? e.g. unit, envtest, manual against vX.Y.Z -->

## Notes for reviewers
<!-- CRD/API changes, RBAC changes, new watches, breaking changes, upgrade path.
     Delete if not applicable. -->

## Checklist
- [ ] Tests added/updated
- [ ] No breaking changes (or upgrade path documented above)

[trevex]

We should also add "readable commit history (squashed, rebased as required)"