Add clarifying note to PR template

[lukastaegert]

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

List any relevant issue numbers:

Description

This will add a comment to the PR template prompting collaborators to list resolved issues as resolves #<issue number> so that Github can auto-close the issues when the PR is merged.

[github-actions]

Thank you for your contribution! ❤️

You can try out this pull request locally by installing Rollup via

npm install rollup/rollup#clarify-pr-template

or load it into the REPL:
https://rollupjs.org/repl/?pr=4128

[codecov]

Codecov Report

Merging #4128 (3dc5851) into master (592b9fe) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4128   +/-   ##
=======================================
  Coverage   98.12%   98.12%           
=======================================
  Files         201      201           
  Lines        7079     7079           
  Branches     2071     2071           
=======================================
  Hits         6946     6946           
  Misses         64       64           
  Partials       69       69           

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