docs: add install instructions for unsigned DMG

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: phrontizo

README.md

@@ -34,6 +34,18 @@ macOS has no built-in Markdown preview. QuickLook shows `.md` files as plain tex
 - **Line numbers** — for easy reference
 - **Dark mode** — adapts to system appearance
 
+## Install
+
+Download the latest DMG from [Releases](https://github.com/phrontizo/QuickMark/releases), open it, and drag QuickMark to Applications.
+
+The app is not signed with an Apple Developer certificate, so macOS will quarantine it. Remove the quarantine attribute before launching:
+
+```bash
+xattr -d com.apple.quarantine /Applications/QuickMark.app
+```
+
+Then open QuickMark and follow the prompts to enable the extensions in System Settings.
+
 ## Requirements
 
 - macOS 14.0+