Add README file for http_server package.

pkg/http_server/README.md

+Library of HTTP server classes.
+
+This package contains a set of high-level classes that, together with
+HttpServer, makes is easy to provide content through HTTP servers.
+
+## Virtual Directory
+
+The VirtualDirectory class makes it possible to easy serve static content from
+the drive. It supports:
+
+ *  Range-based request, making it possible to pause/resume downloads and stream
+    videos.
+ *  If-Modified-Since based caching.
+ *  Automatic GZip-compression of content.
+ *  Ability to follow links within, either throughout the system or in a jailed
+    root.
+ *  Optional directory listing.
+
+ The following example shows how to set up a Virtual Directory of a given path
+
+    var virtualDirectory = new VirtualDirectory('/var/www/');
+    virtualDirectory.serve(new HttpServer('0.0.0.0', 8080));
+
+See [VirtualDirectory](
+http://api.dartlang.org/docs/http_server/VirtualDirectory.html)
+for more info about how to customize the class.
+
+## Virtual Host
+
+The VirtualHost class makes it possible to serve multiple hosts on the same
+address, by using the `Host` field of the incoming requests. It also provides
+the ability to work on wildcards for sub-domains.
+
+    var virtualHost = new VirtualHost(server);
+    // Filter out on a specific host
+    var stream1 = virtualServer.addHost('static.myserver.com');
+    // Wildcard for any other sub-domains.
+    var stream2 = virtualServer.addHost('*.myserver.com');
+    // Requets not matching any hosts.
+    var stream3 = virtualServer.unhandled;
+
+See [VirtualHost](
+http://api.dartlang.org/docs/http_server/VirtualHost.html)
+for more information.