Mention potential gzip incompatibility in release notes (#13653)

Change-Id: I11f2e4095ac106e985e2add283c7d36d04cd07e2

WebContent/release-notes.html

@@ -109,6 +109,8 @@
         <h3 id="incompatible">Incompatible or Behavior-altering Changes in @version-minor@</h3>
         <ul>
             <li>Push path has been changed from /PUSH/ to /PUSH to be compatible with JSR 356.</li>
+            <li>Widgetset files and other pre-compressed resources are sent as gzip to compatible browsers.
+                This may interfere with custom response compression solutions that do not respect the Content-Encoding response header.</li>
         </ul>
         <h3 id="knownissues">Known Issues and Limitations</h3>
         <ul>