ADD: Note on how to setup dns hostname resolution and why it is necessary to...

ADD: Note on how to setup dns hostname resolution and why it is necessary to make the software run properly.

doc/install/installation.md

@@ -286,6 +286,8 @@ GitLab Shell is an SSH access and repository management software developed speci
 
 **Note:** If you want to use HTTPS, see [Using HTTPS](#using-https) for the additional steps.
 
+**Note:** Make sure your hostname can be resolved on the machine itself by either a proper DNS record or an additional line in /etc/hosts ("127.0.0.1  hostname"). This might be necessary for example if you set up gitlab behind a reverse proxy. If the hostname cannot be resolved, the final installation check will fail with "Check GitLab API access: FAILED. code: 401" and pushing commits will be rejected with "[remote rejected] master -> master (hook declined)".
+
 ### Initialize Database and Activate Advanced Features
 
     sudo -u git -H bundle exec rake gitlab:setup RAILS_ENV=production