Remove the .NET implementation. (#1477)

This commit removes the .NET implementation from Wasmtime.

It now exists at https://github.com/bytecodealliance/wasmtime-dotnet.

Also updates the Wasmtime book to include information about using Wasmtime from
.NET.

docs/lang-dotnet.md

@@ -1,3 +1,85 @@
 # Using WebAssembly from .NET
 
-... more coming soon
+The [Wasmtime](https://www.nuget.org/packages/Wasmtime) NuGet package can be used to
+programmatically interact with WebAssembly modules.
+
+This guide will go over adding Wasmtime to your project and demonstrate a simple
+example of using a WebAssembly module from C#.
+
+Make sure you have a [.NET Core SDK 3.0 SDK or later](https://dotnet.microsoft.com/download)
+installed before we get started!
+
+## Getting started and simple example
+
+Start by creating a new .NET Core console project:
+
+```text
+$ mkdir gcd
+$ cd gcd
+$ dotnet new console
+```
+
+Next, add a reference to the Wasmtime NuGet package to your project:
+
+
+```text
+$ dotnet add package --version 0.15.0-preview1 wasmtime
+```
+
+Copy this example WebAssembly text module into your project directory as `gcd.wat`.
+
+```wat
+{{#include ../examples/gcd.wat}}
+```
+
+This module exports a function for calculating the greatest common denominator of two numbers.
+
+Replace the code in `Program.cs` with the following:
+
+```c#
+using System;
+using Wasmtime;
+
+namespace Tutorial
+{
+    class Program
+    {
+        static void Main(string[] args)
+        {
+            using var host = new Host();
+            using var module = host.LoadModuleText("gcd.wat");
+
+            using dynamic instance = host.Instantiate(module);
+            Console.WriteLine($"gcd(27, 6) = {instance.gcd(27, 6)}");
+        }
+    }
+}
+```
+
+Run the .NET core program:
+
+```text
+$ dotnet run
+```
+
+The program should output:
+
+```text
+gcd(27, 6) = 3
+```
+
+
+If this is the output you see, congrats! You've successfully ran your first
+WebAssembly code in .NET!
+
+## More examples and contributing
+
+The [.NET embedding of Wasmtime repository](https://github.com/bytecodealliance/wasmtime-dotnet)
+contains the source code for the Wasmtime NuGet package.
+
+The repository also has more [examples](https://github.com/bytecodealliance/wasmtime-dotnet/tree/master/examples)
+as well.
+
+Feel free to browse those, but if you find anything missing don't
+hesitate to [open an issue](https://github.com/bytecodealliance/wasmtime-dotnet/issues/new) and let us
+know if you have any questions!