[dns] README with basic and cached usage

Author: igorw

README.md

@@ -0,0 +1,44 @@
+# Dns Component
+
+Async DNS resolver.
+
+The main point of the DNS component is to provide async DNS resolution.
+However, it is really a toolkit for working with DNS messages, and could
+easily be used to create a DNS server.
+
+## Basic usage
+
+The most basic usage is to just create a resolver through the resolver
+factory. All you need to give it is a nameserver, then you can start resolving
+names, baby!
+
+    $loop = React\EventLoop\Factory::create();
+    $factory = new React\Dns\Resolver\Factory();
+    $resolver = $factory->create('8.8.8.8', $loop);
+
+    $resolver->resolve('igor.io', function ($ip) {
+        echo "Host: $ip\n";
+    });
+
+But there's more.
+
+## Caching
+
+You can cache results by configuring the resolver to use a `CachedExecutor`:
+
+    $loop = React\EventLoop\Factory::create();
+    $factory = new React\Dns\Resolver\Factory();
+    $resolver = $factory->createCached('8.8.8.8', $loop);
+
+    $resolver->resolve('igor.io', function ($ip) {
+        echo "Host: $ip\n";
+    });
+
+    ...
+
+    $resolver->resolve('igor.io', function ($ip) {
+        echo "Host: $ip\n";
+    });
+
+If the first call returns before the second, only one query will be executed.
+The second result will be served from cache.