Add a Server interface.

README.md

 ## Web Server Middleware for Dart
 
 [![Build Status](https://travis-ci.org/dart-lang/shelf.svg?branch=master)](https://travis-ci.org/dart-lang/shelf)
 [![Coverage Status](https://coveralls.io/repos/dart-lang/shelf/badge.svg?branch=master)](https://coveralls.io/r/dart-lang/shelf)
 
 ## Introduction
 
 **Shelf** makes it easy to create and compose **web servers** and **parts of web
 servers**. How?
 
@@ skipping 130 matching lines @@
 /// [runZoned] and pass them to [onError].
 catchTopLevelErrors(callback(), void onError(error, StackTrace stackTrace)) {
   if (Zone.current.inSameErrorZone(Zone.ROOT)) {
     return runZoned(callback, onError: onError);
   } else {
     return callback();
   }
 }
 ```
 
+An adapter that knows its own URL should provide an implementation of the
+[`Server`][server] interface.
+
+[server]: http://www.dartdocs.org/documentation/shelf/latest/index.html#shelf/shelf@id_Server
+
 ## Inspiration
 
 * [Connect](http://www.senchalabs.org/connect/) for NodeJS.
     * Read [this great write-up](http://howtonode.org/connect-it) to understand
       the overall philosophy of all of these models.
 * [Rack](http://rack.github.io/) for Ruby.
 * [WSGI](http://legacy.python.org/dev/peps/pep-3333/) for Python.