Update PULL_REQUEST_TEMPLATE.md to be more clear

[jglovier]

We are still getting lots of pull requests with unformatted and unhelpful descriptions. Perhaps the template content is unclear. This PR helps to address that.

Changes proposed in this pull request:

• clarify the referenced issue number line
• add placeholder items to proposed changes
• add a note clarifying purpose

cc @HospitalRun/core-maintainers

[jkleinsc]

@jglovier looks good, but I would slightly change the note:
"Note: pull requests without proper descriptions may simply be closed without further discussion. We appreciate your contributions, but need to know what you are offering in clearly described format. Thanks! (you can delete this text)"

[jglovier]

@jkleinsc good call. 👍 Fixed.