Add docs for the issue closing pattern.

doc/README.md

@@ -19,6 +19,7 @@
 - [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.
+- [Issue closing](customization/issue_closing.md) Customize how to close an issue from commit messages.
 
 ## Contributor documentation
 

doc/customization/issue_closing.md

+# Issue closing pattern
+
+By default you can close issues from commit messages by saying 'Closes #12' or 'Fixed #101'.
+
+If you want to customize the message please do so in [gitlab.yml](https://gitlab.com/gitlab-org/gitlab-ce/blob/73b92f85bcd6c213b845cc997843a969cf0906cf/config/gitlab.yml.example#L73)

doc/customization/welcome_message.md

+# Customize the complete sign-in page (GitLab Enterprise Edition only)
+
+Please see [Branded login page](http://doc.gitlab.com/ee/customization/branded_login_page.html)
+
 # 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