Fix README (#23)

The README.md previously file had instructions for how to emit images to
disk. Those instructions were inaccurate and it's hard to make them
accurate for all runtimes. Changed it to use in memory execution of the
assembly.

Author: jaredpar

README.md

@@ -16,15 +16,16 @@ for `netstandard2.0`, `netcoreapp3.1` and `net5.0`. These can be easily
 integrated into the existing Roslyn APIs.
 
 ```c#
-using Microsoft.CodeAnalysis;
 using Microsoft.CodeAnalysis.CSharp;
 using Basic.Reference.Assemblies;
+using System.IO;
+using System.Reflection;
 
 var code = @"
 using System;
-static class Program
+public static class Example
 {
-    static void Main()
+    public static void Main()
     {
         var tuple = (Part1: ""hello"", Part2: ""world"");
         Console.WriteLine($""{tuple.Part1} {tuple.Part2}"");
@@ -38,8 +39,13 @@ var compilation = CSharpCompilation
         new[] { CSharpSyntaxTree.ParseText(code) },
         references: ReferenceAssemblies.Net50);
 
-using var fileStream = new FileStream(@"p:\temp\helloworld.exe", FileMode.Create, FileAccess.ReadWrite);
-var emitResults = compilation.Emit(fileStream);
+using var stream = new MemoryStream();
+var emitResults = compilation.Emit(stream);
+stream.Position = 0;
+var assembly = Assembly.Load(stream.ToArray());
+var method = assembly.GetType("Example").GetMethod("Main");
+method.Invoke(null, null); // Prints "Hello World"
+
 ```
 
 This package also adds extensions methods for easily retargeting `Compilation`