add documentation for emailing from within GitLab

doc/README.md

@@ -20,6 +20,7 @@
 - [Update](update/README.md) Update guides to upgrade your installation.
 - [Branded login page](customization/branded_login_page.md) Change the login page appearance for your GitLab instance.
 - [Git Hooks](git_hooks/git_hooks.md) Advanced push rules for your project.
+- [Email](tools/email.md) Email GitLab users from GitLab
 
 ## Contributor documentation
 

doc/tools/email.md

+# Email from GitLab
+
+As a GitLab administrator you can email GitLab users from within GitLab.
+
+In the administrator interface, go to `Users`. Here you will find the button to email users:
+
+![admin users](email1.png)
+
+Here you can simply compose an email.
+
+![compose an email](email2.png)
+
+Which will be sent to all users or users of a chosen group or project.
+
+![recipients](email3.png)