If you need a wiki page for you project but do not want it in the repo with you code, GitLab offers wiki for each GitLab project. Each wiki is a separate git repo which can be created through the web interface or through git.
To access wiki for a project, look on the left side of the project page under Wiki.
GitLab wiki supports; Markdown, Rdoc, AsciiDoc and Org.
For more information visit GitLab Wiki Pages.