further customization of Nginx defaults to facilitate performance tuning

Adding a number of additional Nginx variables to help with performance tuning, roughly per this thread:
https://www.digitalocean.com/community/tutorials/how-to-optimize-nginx-configuration

I've left the default values for :client_max_body_size ("4G") and :keepalive_timeout ("10") that were previously hard-coded.

Also updated the README.md to add a "Performance Tuning" section.

This pull request seems to want to merge everything since I forked because I think you merged my previous pull into a branch other than 'master'.  I started these recent changes after syncing with upstream/master so you can see only 3 files are currently changed.