Make README template customizable

[ashleywillard]

Allows users to specify a README template other than the default template currently defined in packs. Also allows users to specify a path to the readme template, or by default the template should be located at README_TEMPLATE.md. 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. #154 requested the ability to customize the README but was closed due to inactivity.

• TODO: specs
• I bumped the gem version (or don't need to) 💎

[tstannard]

can you change the content to This is the custom template so that it's clear this test is testing something different from the one on 342