add link to forking appendix

Author: brianamarie

book/13_workflow_review_project_github_games.md

@@ -4,6 +4,8 @@ In this section, we will work on a project repository called `github-games`.
 
 A `github-games` repository has been created for you in the githubschool organization. You can access the repository at `https://github.com/githubschool/github-games-username`.
 
+If you're using the Fork and Pull Workflow, [don't forget to look in the appendix for a more thorough explanation](app_fork_workflow.md).
+
 ### Workflow Review: Updating the README.md
 
 Now you will practice the GitHub Flow from beginning to end by updating the link in the README to point to your fork of the repository.