fix: add supported platforms note to Docker Sandboxes section

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

README.md

@@ -28,6 +28,8 @@ curl -fsSL https://nanoclaw.dev/install-docker-sandboxes.sh | bash
 curl -fsSL https://nanoclaw.dev/install-docker-sandboxes-windows.sh | bash
 ```
 
+<p>Currently supported on macOS (Apple Silicon) and Windows (x86). Linux support coming soon.</p>
+
 <p><a href="https://nanoclaw.dev/blog/nanoclaw-docker-sandboxes">Read the announcement →</a></p>
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 </div>