change nuget README file

Author: NiTiSon

public/NUGET_README.md

@@ -8,6 +8,25 @@ without incurring the overhead of creating new string instances for each operati
 This package provides new types:
 + NiTiS.MutableString
 
+## Using
+
+```cs
+using System;
+using NiTiS;
+
+class Example
+{
+    void Main()
+    {
+        MutableString str = "Hello World";
+
+        str.Insert(0, "> ");
+
+        Console.WriteLine(str); // Output: "> Hello World"
+    }
+}
+```
+
 ## Contributing
 Contributions are welcome!
 Please read [CONTRIBUTING.md](https://github.com/NiTiSon/MutableString/blob/stable/CONTRIBUTING.md) before making any contribution to repository.