Complete admin documentation to explain how to use UFORGE_PROXY_IGNORED

[sohnoun]

Should add a warning to http://docs.usharesoft.com/projects/appcenter-admin-guide/en/latest/pages/rebranding/custo-platform.html?#modifying-uforge-webservice-url
To explain that UFORGE_URL and UFORGE_PROXY_IGNORED must be consistent