Add branded login page documentation.

doc/ee_features/branded_login_page.md

+# Branded Login Page
+
+GitLab Enterprise Edition offers a way to put your company recognizible identity on the login page of your GitLab server.
+
+By default, Enterprise Edition page shows GitLab logo and description
+
+![default_login_page](branded_login_page/default_login_page.png)
+
+## Changing the appearance of the login page
+
+Navigate to the ![admin area](branded_login_page/admin_area.png) and go to the Appearance page.
+
+Fill in the required details like Title, Description and upload the company logo.
+
+![appearance](branded_login_page/appearance.png) 
+
+After saving the page, your GitLab login page will have the details you filled in:
+
+![company_login_page](branded_login_page/company_login_page.png)
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