add flags instructions

Author: fergald

explainers/permissions-policy-deprecate-unload.md

@@ -313,6 +313,30 @@ The currernt proposal makes this a tempoarary state.
 
 No other concerns are known.
 
+# Testing with Chrome
+
+As of [117.0.5924.2](https://chromiumdash.appspot.com/commit/2cf526285beacdfebe2251a20ad5550b3487a277),
+flags are available to control this deprecation.
+To force the depraction,
+you can start Chrome with
+
+```
+--enable-features=PermissionsPolicyUnload --disable-features=DeprecateUnloadByUserAndOrigin
+```
+
+After that you can open a devtools window
+and try
+
+```
+addEventListener("unload", () => {})
+```
+
+You should see
+
+```
+[Violation] Permissions policy violation: unload is not allowed in this document.
+```
+
 # [Self-Review Questionnaire: Security and Privacy](https://w3ctag.github.io/security-questionnaire/)
 
 01.  What information might this feature expose to Web sites or other parties,