add first draft of a pull request template

.github/PULL_REQUEST_TEMPLATE.md

+## Purpose
+_Briefly describe the new feature(s) included in this pull request.
+If this addresses an open GitHub Issue, mention the issue number, e.g. with `fixes #221`_
+
+## Implementation Notes
+_Provide any relevant details about how the feature is implemented,
+how correctness was verified, how other features in LAMMPS are affected_
+
+## Post Submission Checklist
+Please check the fields as they are completed
+- [ ] The feature or features in the pull request is complete
+- [ ] Suitable new documentation files and/or updates to the existing docs are included
+- [ ] One or more example input decks are included
+- [ ] The source code follows the LAMMPS formatting guidelines
+
+## Further Information, Files, and Links
+_Put any additional information here, attach relevant text or image files and URLs to external sites_
+
+