feat: pull_request_template

[wesleyboar]

Overview

Add a template for pull request descriptions.

Changes

• added pull_request_template.md

Testing / UI

https://github.com/DesignSafe-CI/DS-User-Guide/blob/e17c3417/.github/pull_request_template.md

Notes

Same as is used on:

• https://github.com/TACC/Core-CMS
• https://github.com/TACC/Core-Styles
• https://github.com/TACC/Core-Portal
• https://github.com/TACC/Core-CMS-Custom
• https://github.com/TACC/TACC-Docs