Update samples for readonly (#133)

Move existing snippets from docs repo.
Add samples for new uses of readonly.

Author: BillWagner

snippets/csharp/keywords/Program.cs

@@ -14,6 +14,8 @@ static void Main(string[] args)
             FixedKeywordExamples.Examples();
             Console.WriteLine("=================    Iteration Keywords Examples ======================");
             IterationKeywordsExamples.Examples();
+            Console.WriteLine("=================    readonly Keyword Examples ======================");
+            ReadonlyKeywordExamples.Examples();
         }
     }
 }

snippets/csharp/keywords/ReadonlyKeywordExamples.cs

@@ -0,0 +1,89 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+
+namespace keywords
+{
+    // <SnippetReadonlyField
+    class Age
+    {
+        readonly int year;
+        Age(int year)
+        {
+            this.year = year;
+        }
+        void ChangeYear()
+        {
+            //year = 1967; // Compile error if uncommented.
+        }
+    }
+    // </SnippetReadonlyField>
+
+    // <SnippetInitReadonlyField>
+    class SampleClass
+    {
+        public int x;
+        // Initialize a readonly field
+        public readonly int y = 25;
+        public readonly int z;
+
+        public SampleClass()
+        {
+            // Initialize a readonly instance field
+            z = 24;
+        }
+
+        public SampleClass(int p1, int p2, int p3)
+        {
+            x = p1;
+            y = p2;
+            z = p3;
+        }
+
+        public static void Usage()
+        {
+            SampleClass p1 = new SampleClass(11, 21, 32);   // OK
+            Console.WriteLine($"p1: x={p1.x}, y={p1.y}, z={p1.z}");
+            SampleClass p2 = new SampleClass();
+            p2.x = 55;   // OK
+            Console.WriteLine($"p2: x={p2.x}, y={p2.y}, z={p2.z}");
+        }
+        /*
+         Output:
+            p1: x=11, y=21, z=32
+            p2: x=55, y=25, z=24
+        */
+    }
+    // </SnippetInitReadonlyField>
+
+    // <SnippetReadonlyStruct>
+    public readonly struct Point
+    {
+        public double X { get; }
+        public double Y { get; }
+
+        public Point(double x, double y) => (X, Y) = (x, y);
+
+        public override string ToString() => $"({X}, {Y})";
+    }
+    // </SnippetReadonlyStruct>
+
+    public static class ReadonlyKeywordExamples
+    {
+        public static void Examples()
+        {
+            SampleClass.Usage();
+            ReadonlyRefReturns();
+        }
+
+        // <SnippetReadonlyReturn>
+        private static readonly Point origin = new Point(0, 0);
+        public static ref readonly Point Origin => ref origin;
+        // </SnippetReaodnlyReturn>
+
+        private static void ReadonlyRefReturns()
+        {
+            Console.WriteLine(Origin);
+        }
+    }
+}