Create mentoring.md for Beer Song in C#

tracks/csharp/exercises/beer-song/beer-song/mentoring.md

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+## Mentoring
+The student would normally go first using the if/else and if/else if statements.
+Suggesting the use of case statements first will be optimal and instructive. 
+Learning material: [Switch statements ](https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/statements/selection-statements)
+
+## The Exemplar Solution
+Try and guide the student towards this solution.
+```csharp
+public static class BeerSong
+{
+    public static string Recite(int bottles, int takeDown)
+    {
+        string verse = "";
+
+        for (int i = takeDown; i > 0; i--)
+        {
+            switch (bottles)
+            {
+                case 0:
+                    verse += "No more bottles of beer on the wall, no more bottles of beer.\n" +
+                        "Go to the store and buy some more, 99 bottles of beer on the wall.";
+                    break;
+                case 1:
+                    verse += "1 bottle of beer on the wall, 1 bottle of beer.\n" +
+                        "Take it down and pass it around, no more bottles of beer on the wall.";
+                    break;
+                case 2:
+                    verse += "2 bottles of beer on the wall, 2 bottles of beer.\n" +
+                        "Take one down and pass it around, 1 bottle of beer on the wall.";
+                    break;
+                default:
+                    verse += $"{bottles} bottles of beer on the wall, {bottles} bottles of beer.\n" +
+                        $"Take one down and pass it around, {bottles - 1} bottles of beer on the wall.";
+                    break;
+            }
+            if (i != 1)
+                verse += "\n\n";
+
+            bottles--;
+        }
+        return verse;
+    }
+}
+```