Add empty pull_request_template.md to enable PR template chooser

[EwoutH]

We added two pull request templates in #2217, but those never showed up in the PR template chooser. Apparently a pull_request_template.md in .github is always needed, which can be blank.

See

• https://docs.github.com/en/communities/using-templates-to-encourage-useful-issues-and-pull-requests/creating-a-pull-request-template-for-your-repository
• https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/using-query-parameters-to-create-a-pull-request

Directly merging since that's the only way to see if this works.

[EwoutH]

Okay, apparently this does not exist (yet). In classical GitHub fashion, there's an issue open for over a year with over a hundred upvotes that's never once answered by any GitHub staff:

https://github.com/orgs/community/discussions/4620

[Corvince]

Just FYI: I once created a PR through vscode and only (or at least) there did our templates show up.

[EwoutH]

Oh that's interesting (and useful). Curious how most people create Pull requests, through:

• their IDEA
• the git command line
• a git GUI (like GitHub Desktop)
• the github.com interface

I now added a hacky workaround, that works somewhat for the GitHub interface. The default pull_request_template.md now says:

Thanks for opening a PR! Please click the `Preview` tab and select a PR template:

- [🐛 Bug fix](?expand=1&template=bug.md)
- [🛠 Feature/enhancement](?expand=1&template=feature.md)

Which results in clickable links in your PR that you can use to directly get a pull request template.

But this might interfere with IDE implementations of this feature, like VS code.

Feel free to give feedback on this, if this interferes more than it adds we can remove it again.