update instructions for configuring nginx behind ELB, add for sinatra

[bmishkin]

I recently configured a sinatra app using rack-ssl-enforcer using an ELB with only port 80 terminated on the server(s) in nginx. Wanted to contrib my updates => README.

changes:

• proxy_set_header X-Forwarded-Proto $http_x_forwarded_proto; instead of $scheme; - because $scheme is already set to http by the time requests reach nginx. We want to forward on the $http_x_forwarded_proto set by the ELB.
• Added one-line example for sinatra.
• Reformatted nginx example block.

[tobmatth]

Thanks for your contribution, merged.