Update PULL_REQUEST_TEMPLATE.md to include issue cross-reference.

[chahak13]

This changes the PR template to prompt/remind the user to link the issue it resolves. This is to make sure that it reduces duplicate PRs, as having a back link would make it easier to know that there's work being done on a particular issue.

PR Summary

PR Checklist

Documentation and Tests

• [N/A] Has pytest style unit tests (and pytest passes)
• [N/A] Documentation is sphinx and numpydoc compliant (the docs should build without error).
• [N/A] New plotting related features are documented with examples.

Release Notes

• [N/A] New features are marked with a .. versionadded:: directive in the docstring and documented in doc/users/next_whats_new/
• [N/A] API changes are marked with a .. versionchanged:: directive in the docstring and documented in doc/api/next_api_changes/
• [N/A] Release notes conform with instructions in next_whats_new/README.rst or next_api_changes/README.rst

[story645]

I'm wondering also if we can use githubs form schema and make this a required text area.

[chahak13]

I'm wondering also if we can use githubs form schema and make this a required text area.

It doesn't have to be necessary because that might hamper just normal MNT PRs or something that's not related to the issue.