Update README.md

Update frame-ancestors note.

README.md

@@ -58,10 +58,10 @@ location /external_api.js {
 }
 ```
 
-The following additional HTTP headers are known not to work with the Electron App:
+:warning: The following additional HTTP headers are known to break the Electron App:
 
 ```
-Content-Security-Policy "frame-ancestors 'none'";
+Content-Security-Policy "frame-ancestors [looks like any value is bad]";
 X-Frame-Options "DENY";
 ```