update nginx+fcgi deployment guide for PSGI compat

update nginx+fcgi deployment guide for PSGI compat

SCRIPT_NAME must not be '/' according to the PSGI spec, plackup without
-E deployment will load the Lint middleware, which will complain about
this and not allow the app to start.

Update the SCRIPT_NAME to '' for root apps and include a note about how
to do this if using the distributed /etc/nginx/fastcgi_params.