add a description of nginx.conf

it's not intented to be complete, merely guide users unfamiliar with
nginx.

fixes #39

doc/nginx-conf.md

@@ -0,0 +1,55 @@
+[Nginx Server Configs homepage](https://github.com/h5bp/server-configs-nginx)
+ | [Documentation table of contents](TOC.md)
+
+# Nginx.conf
+
+The `nginx.conf` file is the main config file for nginx, which either defines
+or includes the whole configuration for the server.
+
+When editing or defining an nginx configuration file - take care to note in
+which [context](http://nginx.org/en/docs/beginners_guide.html#conf_structure)
+a directive applies.
+
+Below are some notes on a few of the more important/commonly-edited directives.
+For detailed information on any particular directive, please see
+[the official documentation](http://nginx.org/en/docs/).
+
+## user
+
+The [user directive](http://nginx.org/en/docs/ngx_core_module.html#user)
+indicates which user the server will run as. This is typically a user
+specifically for web usage such as "www" "www-data".
+
+The webserver user, and file permissions for any application, should be defined/chosen
+following [the principle of least privilege](http://en.wikipedia.org/wiki/Principle_of_least_privilege)
+i.e., sufficient priveleges to function correctly but no more than that.
+
+## worker_processes
+
+The [worker_processes directive](http://nginx.org/en/docs/ngx_core_module.html#worker_processes)
+broadly defines the number of connections nginx can process.
+
+As of version 1.2.5+ and 1.3.8+, nginx supports the value "auto" which will
+automatically detect an appropriate value. In earlier versions setting to
+the number of CPUs is a good default/starting point.
+
+## error_log
+
+The [error_log directive](http://nginx.org/en/docs/ngx_core_module.html#error_log)
+can be defined/overriden in any context. The directive in the main context
+defines the log file to use unless otherwise overriden (at http, server or location
+level). This must point to a location writable to the webserver user.
+
+The location for log files varies depending on the operating system; one technique
+to allow the config files to be ignorant of this is to symlink `/etc/nginx/logs`
+to where you would like log files to be located. e.g.:
+
+    mkdir /var/log/nginx
+    cd /etc/nginx
+    ln -s /var/log/nginx logs
+
+## pid
+
+The [pid directive](http://nginx.org/en/docs/ngx_core_module.html#pid) is used
+by nginx to store the process id of the main process. This must point to a writable
+location.