[puter] readme: add warning

Author: velzie

README.md

@@ -15,7 +15,8 @@
     ·
     <a href="https://twitter.com/HeyPuter">X</a>
 </p>
-
+> [!WARNING]
+> Browser.js does not currently have site isolation! Do not use it for sensitive data!
 <h3 align="center"><img style="border-radius:5px;" alt="screenshot" src="./assets/screenshot.png"></h3>
 
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