Add C# P/INVOKE wrapper and client/server example

examples/csharp/README.md

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+ZeroTier Sockets for C# .NET (Work In Progress)
+=====
+
+This library is a re-implementation of the .NET socket class ([System.Net.Sockets.Socket](https://docs.microsoft.com/en-us/dotnet/api/system.net.sockets.socket)) built atop ZeroTier's SDK using P/INVOKE and is designed to be a direct drop-in replacement. The library consists of three main objects: `ZeroTier.Node`, `ZeroTier.Event`, and `ZeroTier.Socket`. No code change is required in your application beyond a small snippet of startup code, renaming `Socket` to `ZeroTier.Socket` (where applicable) and handling a smattering of events.
+
+
+tl;dr:
+
+```
+using System.Net.Sockets;
+using ZeroTier;
+
+void myCallback(ZeroTier.Event e)
+{
+	Console.WriteLine("{0} ({1})", e.EventCode, e.EventName);
+}
+...
+
+ZeroTier.Node node = new ZeroTier.Node("path", myCallback, 9991);     	
+
+node.Start();	
+node.Join(0xc287ac0b42a6fb4c);
+
+...
+
+ZeroTier.Socket sock = new ZeroTier.Socket(ipAddr.AddressFamily, SocketType.Stream, ProtocolType.Tcp);
+
+sock.Connect(remoteEndPoint);
+
+...
+
+node.Stop();
+```
+See [example.cs](./example.cs) for a complete client/server implementation.
+
+## Building and running the example
+
+```
+make debug|release
+./example.exe
+```
+
+## Development notes
+
+The SWIG interface file `zt.i` is only present for historical reference purposes. SWIG generates a ton of unnecessary boilerplate code which is hard to completely prevent using hints. You can generate a new wrapper for yourself using `swig -c++ -csharp -dllimport "./libzt.so" zt.i` but I would not recommend doing so unless you know what you're in for.