Make README template customizable

[ashleywillard]

Allows users to specify a README template other than the default template currently defined in pks. Also allows users to specify a path to the readme template, or by default the template should be located at README_TEMPLATE.md. The generated README.md file currently contains the following text:

Welcome to `{}`!

If you're the author, please consider replacing this file with a README.md, which may contain:
- What your pack is and does
- How you expect people to use your pack
- Example usage of your pack's public API and where to find it
- Limitations, risks, and important considerations of usage
- How to get in touch with eng and other stakeholders for questions or issues pertaining to this pack
- What SLAs/SLOs (service level agreements/objectives), if any, your package provides
- When in doubt, keep it simple
- Anything else you may want to include!

README.md should change as your public API changes.

See https://github.com/rubyatscale/pks#readme for more info!

Some organizations may want to use their own README template. rubyatscale/packs#154 requested the ability to customize the README in packs, but was closed due to inactivity.

[perryqh]

Run cargo fmt to pass the CLI/Lints step