I was planning to prepare some documentation, but not sure when I'll be find time for that :-(
I am testing updated version of ISPConfig patch and will try to release some additional information regarding building/installing/configuring nginx/php-fpm with ISPConfig, soon.
At the moment there is very simple readme inside the patch, I've posted earlier.
I can't post patches here, since forum has 100k limit. I'll check my options and post back.
Contact me for clarifications.