GitHub repo lacks community guidelines

[benlk]

More about GitHub's community guidelines features: https://help.github.com/en/articles/creating-a-default-community-health-file-for-your-organization

Adding guidelines to the repo would provide clear guidance on who can propose edits and make pull requests, removing the need for questions like https://github.com/Automattic/Co-Authors-Plus/issues/627#issuecomment-472422975

A list of things to add:

• [ ] LICENSE.txt
• [ ] .github/CONTRIBUTING.md https://help.github.com/en/articles/setting-guidelines-for-repository-contributors
• [ ] .github/pull_request_template.md https://help.github.com/en/articles/creating-a-pull-request-template-for-your-repository
• [ ] .github/issue_template.md https://help.github.com/en/articles/creating-issue-templates-for-your-repository and https://help.github.com/en/articles/manually-creating-a-single-issue-template-for-your-repository