Implement a .shed.yml 2 README.md command.

- Use description and long_description to assemble the core of the README.md.
- Use repository url to build lists of tools and links in Markdown.
- Optionally add some basic usage information based on all of this (how to clone, test, serve, update, etc...).

Consider a few more .shed.yml fields to enhance both README and shed recording - such as license information, contributing, etc....

Start by locating the command that processes .shed.yml and the existing README.md generation path. Trace how description, long_description, and repository URL are currently represented, then review related command tests if present. Done means the command assembles the requested Markdown content and handles the proposed optional fields without breaking existing behavior.

Written by the indexing model from the issue text.