feat: Add URL rewrite support documentation (issue #8)

[cjxzdzh]

Issue

Fixes #8

What changed

Added a new section URL 原文获取（可选） to SKILL.md that documents how to use humanizer-zh with a URL instead of raw text.

Details

• 工作流程: Describes the 4-step process (identify URL type → fetch via web_extract/curl → humanize → return result)
• 支持的 URL 来源: Table of supported sources (web pages, GitHub raw files, .txt/.md docs, PDFs) with the fetch tool used for each
• 使用示例: Three concrete examples (web page URL, GitHub raw file URL, PDF URL)
• 无法抓取时的处理: Graceful fallback when URL is inaccessible (ask user to paste raw text)

This is purely documentation — no implementation change required since the fetch behavior is already available via the web_extract and terminal tools built into the skill runtime.