add a document explaining request handling

it's not exactly the same as one might expect

doc/TOC.md

@@ -6,6 +6,7 @@
 
 * [Usage](usage.md) — Overview of the project contents.
 * [Nginx.conf](nginx-conf.md) —General introduction to the main config file.
+* [How Nginx works](how-nginx-works.md) —Understanding nginx, and how it differs from other webservers.
 * [Common problems](common-problems.md) —Dealing with commonly-encounterederrors.
 
 ## Related projects

doc/how-nginx-works.md

@@ -0,0 +1,41 @@
+[Nginx Server Configs homepage](https://github.com/h5bp/server-configs-nginx)
+ | [Documentation table of contents](TOC.md)
+
+# How Nginx works
+
+If you're familiar with any other webserver, some aspects of
+[the way Nginx works](http://nginx.org/en/docs/http/request_processing.html)
+ can cause confusion. This document aims to highlight differences or features
+which may trip up new users.
+
+## Nginx will only use one location block
+
+A [location block (directive)](http://nginx.org/en/docs/http/ngx_http_core_module.html#location)
+defines the behavior for a given request which matches the location url pattern.
+
+It is very important when writing nginx configuration files to understand that
+only one location block will be used by Nginx. When in doubt a useful technique
+to identify which location block is to add a header:
+
+	# Make sure js files are served with a long expire
+	location ~ /something {
+		add_header "section" "something location";
+
+		...
+	}
+
+	location /something-else {
+		add_header "section" "something else location";
+
+		...
+	}
+
+In the headers for a response, the header added from the block which matched
+will be included:
+
+	$ curl -I "http://nginx.h5bp.dev/something"
+	...
+	section: something location
+
+There are some significant concequences to this behavior such as it _not_ being
+possible to build configuration files from small, overlapping, location blocks.