docs(pull_request_template): add merging guidelines

[rubinovitz]

PR Description

PR Checklist

Testing

• [ ] added relevant test coverage
• [x] no tests needed

Docs

• [x] added relevant docs
• [ ] updated relevant sections in the README.md
• [ ] updated relevant docstrings in index.d.ts
• [ ] no docs needed

Typescript

• [ ] added or edited relevant Typescript type declarations
• [x] no type declaration updates needed

[github-actions[bot]]

:tada: This PR is included in version 0.5.1 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot :package::rocket: