To add a PR template it really simple. You just must create a file with the name PULL_REQUEST_TEMPLATE.md and the info/template as you wish then it will be used as follows when a PR be made for the repo where this template was added.
It is a good suggestion to create the directory .github in order to keep there all GitHub docs as README.md, and CONTRIBUTING.md, and ISSUE_TEMPLATE.md, and etc …