chore: add more details on ngrok setup when running smoke tests with user session locally

Author: ytcleon

README.md

@@ -167,7 +167,7 @@ To run a test suite for a type of FT subscriber, add a `user` property to the su
 
 For the test to get the user session tokens from [`next-test-sessions-lambda`](http://github.com/financial-times/next-test-sessions-lambda), it needs to rewrite the URL being tested to an ft.com host. The original URL is set in the `FT-Test-Host` header, which tells `next-router` to proxy the test URL rather than production.
 
-The test output will display the original URL. 
+The test output will display the original URL.
 
 *Options:* `premium`, `standard`, `expired`.
 
@@ -202,11 +202,20 @@ n-test smoke -H https://05bd2344ebca.ngrok.io
 
 Needs to set TEST_SESSIONS_URL (url to [`next-test-sessions-lambda`](http://github.com/financial-times/next-test-sessions-lambda)) and TEST_SESSIONS_API_KEY environment variables when running the tests.
 
+```
+TEST_SESSIONS_API_KEY=<TEST_SESSIONS_API_KEY> TEST_SESSIONS_URL=<TEST_SESSIONS_URL> n-test smoke -H https://05bd2344ebca.ngrok.io
+```
+
+Ngrok has added a warning page to block phishing attack, so an additional header `ngrok-skip-browser-warning` is needed to be truthy to bypass the page, or the warning page is always shown.
+
 *Example*
 ```
 [
   {
 	user: 'premium',
+	header: {
+		'ngrok-skip-browser-warning': 1,
+	},
 	urls: [
 	  '/these-will': 200,
 	  '/run-with-a': 200,