chore(repo): add tips for issue numbers in pr template

[TrickyPi]

Rollup Plugin Name: {name}

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes (bugfixes and features will not be merged without tests)
• no

Breaking Changes?

• yes (breaking changes will not be merged unless absolutely necessary)
• no

If yes, then include "BREAKING CHANGES:" in the first commit message body, followed by a description of what is breaking.

List any relevant issue numbers:

Description

I found some PRs that didn't use resolves or other fields to link the issues it will fix, so I added some tips from https://github.com/rollup/rollup/blob/master/.github/PULL_REQUEST_TEMPLATE.md

[shellscape]

Thanks for the suggestion. I'm not sure we need such a large block dedicated to that, since it historically hasn't been a problem with housekeeping.

[TrickyPi]

Sorry for the late reply. I think the "resolves" field is a huge help for contributors or maintainers. In the issues panel, we can easily know which issues have which PRs will close it, there is similar help in the PRs panel. BTW, some issues have been fixed but not closed yet, e.g. #1465, the "resolves" field will also reduce the occurrence of this situation.