[ci skip] Update CHANGELOG.md

CHANGELOG.md

@@ -8,6 +8,8 @@
 - Upgrade Marp Core to [v4.0.0](https://github.com/marp-team/marp-core/releases/v4.0.0) ([#591](https://github.com/marp-team/marp-cli/pull/591))
   - The slide container of built-in themes became the block element and adopted safe centering
   - Relax HTML allowlist: Allowed a lot of HTML elements and attributes by default
+- Use [the new headless mode of Chrome](https://developer.chrome.com/docs/chromium/headless) while converting by default ([#593](https://github.com/marp-team/marp-cli/pull/593))
+  - You can get back to the old headless mode by setting `PUPPETEER_HEADLESS_MODE=old` env.
 
 ### Added