Update Readme and add ref to samples (#2)

***NO_CI***

Author: oblaise

README.md

@@ -17,6 +17,98 @@ This API implements the UdpClient class with a pattern similar to the official .
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+## Usage
+
+**Important:** Obviously UdpClient requires a working network connection with a valid IP address. Please check the examples with the Network Helpers on how to connect to a network.
+
+The UdpClient class provides simple methods for sending and receiving UDP datagrams on an IP network. The current implementation supports only IPv4. IPv6 isn't supported on nanoFramework currently.
+
+### Samples
+Samples for `UdpClient` are present in the [nanoFramework Sample repository](https://github.com/nanoframework/Samples).
+
+### Remote host
+Because UDP is a connectionless protocol you don't need to establish a remote host connection before sending and receiving data. But you can define a default remote host that will be used for subsequent `Send` method calls. If you establish a default remote host, you cannot specify a different host when sending datagrams. You can define a default remote host with one of the following methods:
+- Create your client using the `UdpClient(string hostname,string remoteport)` constructor.
+- Create an instance and then call the `Connect` method.
+
+### Client usage
+Using `UdpClient` in client mode is pretty easy. You create an UdpClient with one of the constructor, establish or not a default remote host (see above) then you can send and receive message from the network. 
+
+The following code show a typical client server exchange where you first send a message to the server and wait for the server answer:
+
+```C#
+// establish defaut host and port
+UdpClient udpClient = new UdpClient("1.2.3.4", 5000);
+udpClient.Send(Encoding.UTF8.GetBytes("Hello server"));
+
+// receive message
+byte[] buffer = new byte[1024];
+IPEndPoint ipEndpoint = new IPEndPoint(IPAddress.Any, 0);
+int length = udpClient.Receive(buffer, ref ipEndpoint);
+Debug.WriteLine(Encoding.UTF8.GetString(buffer, 0, length));
+```
+
+### Server usage
+Working as server you bind your `UdpClient` on a local port then you wait for messages from clients. As a server you don't know beforehand the IP address of your clients so you shouldn't define any remote host.
+
+The following code show a simple echo server:
+```c#
+// Run echo protocol on port 5000
+UdpClient udpClient = new UdpClient(5000); 
+
+// We limit ourself to a 1024 bytes buffer
+byte[] buffer = new byte[1024];
+IPEndPoint endpointClient = new IPEndPoint(IPAddress.Any, 0);
+
+// We send back every request we get
+while (true)
+{
+    int length = udpClient.Receive(buffer, ref endpointClient);
+    udpClient.Send(buffer,endpointClient);
+}
+```
+
+### Multicast
+If you want to use multicast ensure that you bind your `UdpClient` on the `0.0.0.0` (wilcard) address. If you bind your UdpClient to a specific `IPAddress` you won't receive the multicast datagrams.
+
+Basically for a functional multicast client/server you need to:
+- Create your UdpClient without binding it to a specific address.
+- Join a Multicast group by a call to the JoinMulticastGroup method.
+- Receive datagram sent to that group address with call to `Receive`
+- Send message to the group multicast using `Send`
+- Leave the Multicast group by calling the `DropMulticastGroup` method
+
+The following sample illustrate that basic workflow:
+
+```C#
+// Create your UdpClient without binding it to a specific address
+IPEndPoint iPEndpoint = new IPEndPoint(IPAddress.Any, 5000);
+UdpClient client = new UdpClient(iPEndpoint);
+
+// Join a Multicast group
+IPAddress ipGroupMulticast = IPAddress.Parse("239.255.255.250");
+client.JoinMulticastGroup(ipGroupMulticast);
+
+bool StopListener = false;
+byte[] buffer = new byte[2048];
+while (!StopListener)
+{
+    IPEndPoint remote = new IPEndPoint(IPAddress.Any, 0);
+    int length = client.Receive(buffer, ref remote);
+    string result = Encoding.UTF8.GetString(buffer, 0, length);
+    if (result == "Ping")
+    {
+        buffer = Encoding.UTF8.GetBytes("Present");
+        client.Send(buffer,ipGroupMulticast);
+    }
+    StopListener = (result == "Exit");
+}
+
+// Leave the Multicast group
+client.DropMulticastGroup(ipGroupMulticast);
+```
+If you want to receive your own messages you can enable this by setting the `UdpClient.MulticastLoopback` property to `true`.
+
 ## Feedback and documentation
 
 For documentation, providing feedback, issues and finding out how to contribute please refer to the [Home repo](https://github.com/nanoframework/Home).