Move pull request template to .github subdirectory

[mhucka]

As a general housekeeping measure, I think it would be nicer to put the pull request template .md file into .github/pull_request_template.md instead of letting it clutter up the top-level source directory.

I can do that, if there are no objections.

[fbergmann]

that seems like a good idea.

[mhucka]

So, this is more complicated than expected. I think this has to be done on the stable branch, after the change is done, the GitHub configuration has to be updated. I hesitate doing it on the stable branch. Should I? Or maybe someone else should do it?