Create initial file for Wiki documentation

doc/user/project/wiki.md

+# Wiki
+
+Each project has a Wiki, where you can create a Wiki in the web interface or
+locally, and collaborate on it with your team using the full power of Git.
+
+A permission level of **Guest** is needed to view a Wiki and **Developer** is needed
+to create and edit Wiki pages. Contact a Project Admin if you need access.
+
+## Getting Started: Create the Home Page
+
+1. When viewing your project, click on **Wiki**.
+1. You will be taken to `/home` of your Wiki, this is where users will land
+when they visit your Wiki. You will be  asked to create the home page if it
+doesn't exist.
+1. You only have to fill in the **Content** section and click **Create page**.
+In the web interface the commit message is optional, but the GitLab Wiki is
+based on Git and needs a commit message, so one will be created for you if you
+do not enter one.