add Nginx test

doc/install/installation.md

@@ -316,7 +316,15 @@ Make sure to edit the config file to match your setup:
     # domain name of your host serving GitLab.
     sudo editor /etc/nginx/sites-available/gitlab
 
-**Note:** If you want to use HTTPS, replace the `gitlab` nginx config with `gitlab-ssl`. See [Using HTTPS](#using-https) for all necessary details.
+**Note:** If you want to use HTTPS, replace the `gitlab` Nginx config with `gitlab-ssl`. See [Using HTTPS](#using-https) for all necessary details.
+
+### Test Configuration
+
+Validate your `gitlab` or `gitlab-ssl` Nginx config file with the following command:
+
+    sudo nginx -t
+    
+You should receive `syntax is okay` and `test is successful` messages. If you receive errors check your `gitlab` or `gitlab-ssl` Nginx config file for typos, etc. as indiciated in the error message given.
 
 ### Restart