Add Troubleshooting section to 7.14 to 8.0 Update guide

doc/update/7.14-to-8.0.md

@@ -190,3 +190,12 @@ sudo -u git -H bundle exec rake gitlab:backup:restore RAILS_ENV=production
 ```
 
 If you have more than one backup `*.tar` file(s) please add `BACKUP=timestamp_of_backup` to the command above.
+
+## Troubleshooting
+
+### "You appear to have cloned an empty repository."
+
+If you see this message when attempting to clone a repository hosted by GitLab,
+this is likely due to a missing or misconfigured `gitlab-git-http-server`
+instance. Double-check that you correctly completed [Step 5](#5-install-gitlab-git-http-server)
+to install the daemon and [Step 8](#new-nginx-configuration) to reconfigure Nginx.