add use-cases to email-from-gl (replaces !662)

doc/tools/email.md

 # Email from GitLab
 
+## Overview
+
 As a GitLab administrator you can email GitLab users from within GitLab.
 
+## Use-cases
+
+- Notify your users about a new project, a new feature, or a new product launch.
+- Notify your users about a new deployment, or that will be downtime expected
+for a particular reason.
+
+## Description
+
 In the administrator interface, go to `Users`. Here you will find the button to email users:
 
 ![admin users](email1.png)
@@ -15,6 +25,7 @@ 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.