Add relative URL info

[ci skip]

config.yml.example

@@ -6,8 +6,8 @@
 # GitLab user. git by default
 user: git
 
-# Url to gitlab instance. Used for api calls.
-# Default: http://localhost:8080
+# URL to GitLab instance, used for API calls. Default: http://localhost:8080.
+# For relative URL support read http://doc.gitlab.com/ce/install/relative_url.html
 # You only have to change the default if you have configured Unicorn
 # to listen on a custom port, or if you have configured Unicorn to
 # only listen on a Unix domain socket. For Unix domain sockets use