Communicate package index status

[schlessera]

The WP-CLI package index is currently on hold. However, this is not really communicated, apart from a comment in one of the pull requests: wp-cli/package-index#90 (comment)

To better communicate the state of the package index, and what that means for users we should do one or more of the following:

• Describe the current state and its consequences in the package index's README.md file
• Put up a notice pointing to the README.md section on the package index's frontpage
• Put up a notice pointing to the README.md section on the "Add to the package index" section of the handbook
• Put a warning pointing to the README.md section into a .github/PULL_REQUEST_TEMPLATE.md file

[danielbachhuber]

Closing in favor of #4236