Create pull_request_template.md

Created by: jorana

This is the same template as in https://github.com/AxFoundation/strax/pull/302.

When writing PRs in strax it might be good to add a template for both filing issues and adding PR. This can help keeping the pull requests in a similar format. It might also help authors to include things (like tests, documentation, examples) that would probably be requested by the reviewers.