Add documentation of gzip_static configuration

The attribute was added (https://github.com/miketheman/nginx/commit/aa68b47717b2b ) but it was not documented in the README. I had to dig for it.

I wonder if adding http_gzip_static_module should set `node['nginx']['gzip_static'] = 'yes'` by default? That seems like the principle of least surprise. It took me a while to figure out why my .gz files were not being served even though I had enabled the module.