Add note to documentation.

doc/tools/email.md

@@ -13,3 +13,8 @@ Here you can simply compose an email.
 Which will be sent to all users or users of a chosen group or project.
 
 ![recipients](email3.png)
+
+## Note
+User can choose to unsubscribe from receiving emails from GitLab by following the unsubscribe link from the email.
+Unsubscribing is unauthenticated in order to keep the simplicity of this feature. On usubscribe, user will receive an email notifying that unsubscribe happened.
+Endpoint that provides unsubscribe option is protected by request being rate-limited.