Consider adding a pull_request_template.md to point GitHub contributors to the right place

[noisysocks]

Now that you can ​link pull requests to Trac tickets in Core, we should consider ​adding a pull_request_template.md to trunk which informs contributors who land on the ​wordpress-develop GitHub repository that pull requests must be linked to a Trac ticket.

The template could look something like this:

<!--
Hi there! Thanks for contributing to WordPress!

WordPress uses Trac and SVN for development. Pull requests in this GitHub repository are only used for code review, and must be linked to a ticket in the WordPress Core Trac (https://core.trac.wordpress.org).

See the WordPress Core Handbook for more information: 

LINK TO HANDBOOK PAGE
-->

Trac: <!-- insert a link to the WordPress Core Trac ticket here -->

<!-- insert a short description of your changes here -->

---

**This pull request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See [the WordPress Core Handbook](LINK TO HANDBOOK PAGE) for more details.**

[kraftbj]

Duplicate of #49489 resolved via r47342.

[noisysocks]

Doh, Jonathan beat me to it! 🤦‍♂️ Thanks @kraftbj.