add documentation for custom brand logo in email

doc/README.md

@@ -50,6 +50,7 @@
 - [System hooks](system_hooks/system_hooks.md) Notifications when users, projects and keys are changed.
 - [Update](update/README.md) Update guides to upgrade your installation.
 - [Welcome message](customization/welcome_message.md) Add a custom welcome message to the sign-in page.
+- [Header logo](customization/branded_page_and_email_header.md) Change the logo on the overall page and email header.
 - [Reply by email](administration/reply_by_email.md) Allow users to comment on issues and merge requests by replying to notification emails.
 - [Migrate GitLab CI to CE/EE](migrate_ci_to_ce/README.md) Follow this guide to migrate your existing GitLab CI data to GitLab CE/EE.
 - [Git LFS configuration](workflow/lfs/lfs_administration.md)

doc/customization/branded_page_and_email_header.md

+# Changing the logo on the overall page and email header
+
+Navigate to the **Admin** area and go to the **Appearance** page.
+
+Upload the custom logo (**Header logo**) in the section **Navigation bar**.
+
+![appearance](branded_page_and_email_header/appearance.png)
+
+After saving the page, your GitLab navigation bar will contain the custom logo:
+
+![custom_brand_header](branded_page_and_email_header/custom_brand_header.png)
+
+The GitLab pipeline emails will also have the custom logo:
+
+![custom_email_header](branded_page_and_email_header/custom_email_header.png)