Add Pull Request Template

Github pull request templates  offer a way to standardize a pull request description.

Undescriptive pull request descriptions are common in core, and make understanding of a pull request more difficult. 

Adding descriptions does mean some duplication of information from JIRA, but pull requests description offer a place to document technical details into the reason for change and implementation, all of which is available at a glance when looking at the code changes.

Pull requests templates are currently used by [CiviHR | https://github.com/civicrm/civihr/pulls] and are working well. I will create a PR to add this, with a modified version of the CiviHR template as a starting point.