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. That's not wrong,
+but the idea is guide him to an approach that will improve the readability and conciseness of the code.
+Suggesting the use of case statements first will be optimal and instructive, since the number of cases
+are bigger than a simple if/else.
+When doing this, you can reference the learning material for the switch and highlight the usability of it.
+Learning material: [Switch statements ](https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/statements/selection-statements)
+
+It's important to check if the student is also using clear names for the variables.
+
+The verse string in the exercise is huge and it seems to repeat in every sentence, but as the exercise 
+explains, it's not always equal and that's why we have more than just one or two cases for it.
+With large strings inside the code, instructing the student to break them improves the readability.
+
+Uncle Bob from the "Clean Code" Book recommends that every line should not contain more than 120 characters.
+This can be challenging sometimes, but it's worthy. So in the exemplar solution we format the string 
+using concatenation. 
+```csharp
+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.";
+```
+Show to the student the importance of formatting the code. It helps a lot with the comprehension
+and when they went back to check older code, they won't be so confused with their own code.
+Programmers spend more time reading than writing code.
+
+If they still feel confused about formatting, this article dive deeper into it.
+[Clean Code - Formatting](https://dev.to/caiocesar/clean-code-in-c-part-4-formatting-1b1h)
+
+## The Exemplar Solution
+Guide the student towards a solution closer to this. 
+
+```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;
+    }
+}
+```