docs: update signed webhook usage documentation (#440)

TROUBLESHOOTING.md

@@ -14,6 +14,7 @@ If you can't find a solution below, please open an [issue](https://github.com/se
 * [Versions](#versions)
 * [Environment Variables and Your Twilio SendGrid API Key](#environment)
 * [Viewing the Request Body](#request-body)
+* [Verifying Event Webhooks](#signed-webhooks)
 
 <a name="migrating"></a>
 ## Migrating from v2 to v3
@@ -108,3 +109,14 @@ You can do this right before you call `response, err := client.Send(message)` li
 ```go
 fmt.Println(string(mail.GetRequestBody(message)))
 ```
+
+<a name="signed-webhooks"></a>
+## Signed Webhook Verification
+
+Twilio SendGrid's Event Webhook will notify a URL via HTTP POST with information about events that occur as your mail is processed. [This](https://docs.sendgrid.com/for-developers/tracking-events/getting-started-event-webhook-security-features) article covers all you need to know to secure the Event Webhook, allowing you to verify that incoming requests originate from Twilio SendGrid. The sendgrid-go library can help you verify these Signed Event Webhooks.
+
+You can find the end-to-end usage example [here](helpers/eventwebhook/README.md) and the tests [here](helpers/eventwebhook/eventwebhook_test.go). 
+If you are still having trouble getting the validation to work, follow the following instructions:
+- Be sure to use the *raw* payload for validation
+- Be sure to include a trailing carriage return and newline in your payload
+- In case of multi-event webhooks, make sure you include the trailing newline and carriage return after *each* event