Add README template pathname

[ashleywillard]

We want to allow users to specify a README template other than the default template currently defined in packs. The README_TODO file currently contains the following text:

Welcome to `#{pack_name}`!

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 (which lives in `#{pack_name}/#{ParsePackwerk::DEFAULT_PUBLIC_PATH}`)
  - 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 (note: it is recommended to add ownership in `#{pack_name}/package.yml` under the `owner` metadata key)
  - 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 files are under version control and should change as your public API changes.#{' '}

See #{documentation_link} for more info!

Some organizations may want to use their own README template. This issue requested the ability to customize the README but was closed due to inactivity. With this change, users can define their own README template, README_TEMPLATE.md, in the project root or specify an alternative path in packs.yml.

Corresponding packs change: rubyatscale/packs#165

• TODO: tests
• Bump version

[ashleywillard]

Teal and I discussed this change. While this makes it very convenient to get the path for the README template, it's not necessary for the public API that this pack provides. If someone were using packs-specification but not packs, they wouldn't need this functionality. I'll move this code to either parse-packwerk or packs.