docs: document the styleguide skills plugin in the README

[OmarAlJarrah]

The root README didn't mention the Claude Code skills now living in skills/. This adds:

• a Claude Code skills section with the one-time install commands and a pointer to skills/README.md for the full index (kept there rather than duplicated here),
• a bullet in "This repository holds" so the plugin is discoverable from the top,
• a step in "Adding a new language guide" to add and register a matching skill, so new guides don't ship without one.

The guides remain the source of truth; the section makes clear the skills distill them and never override them.