Let's have some detailed instructions explaining how to create an extension.

I think it could be a variation of this https://github.com/scaffold-eth/create-eth/blob/main/contributors/RFC-extensions.md for external extensions

• Folder structure (extension folder, etc)
• How new files are folders are copied.
• Special files (.gitignore, etc)
• Dependencies (root package.json / package folder package.json)
• .template / .args files. List them, how they work, etc.

This could live here but also on the create-eth repo.