docs: add English callout line in Chinese README pointing to English version

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

README.md

@@ -21,6 +21,7 @@
 Agent 写小说。写、审、改，全程接管。
 
 **现已支持原生英文创作** — `--lang en` 即可切换，详见 [English README](README.en.md)。
+**Native English support** — Set `--lang en` to write in English. See [English README](README.en.md) for details.
 
 ## 快速开始