Document the extra.sign_in_text setting for CE

doc/README.md

@@ -18,6 +18,7 @@
 - [System hooks](system_hooks/system_hooks.md) Let GitLab notify you when certain management tasks need to be carried out.
 - [Security](security/README.md) Learn what you can do to further secure your GitLab instance.
 - [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.
 ## Contributor documentation

doc/customization/welcome_message.md

+# Add a welcome message to the sign-in page (GitLab Community Edition)
+
+It is possible to add a markdown-formatted welcome message to your GitLab
+sign-in page. Users of GitLab Enterprise Edition should use the [branded login
+page feature](/ee/customization/branded_login_page.html) instead.
+
+## Omnibus-gitlab example
+
+In `/etc/gitlab/gitlab.rb`:
+
+```ruby
+gitlab_rails['extra_sign_in_text'] = <<'EOS'
+# ACME GitLab
+Welcome to the [ACME](http://www.example.com) GitLab server!
+EOS
+```
+
+Run `sudo gitlab-ctl reconfigure` for changes to take effect.
+
+## Installation from source
+
+In `/home/git/gitlab/config/gitlab.yml`:
+
+```yaml
+# snip
+production:
+  # snip
+  extra:
+    sign_in_text: |
+      # ACME GitLab
+      Welcome to the [ACME](http://www.example.com) GitLab server!
+```      
+
+Run `sudo service gitlab reload` for the change to take effect.