Made Add function, Added Tests. Added comments to int Tests

FlexArray/FlexArrayString.cs

@@ -27,4 +27,21 @@ public class FlexArrayString
         var size = Math.Max(0, initialSize ?? DefaultStartSize);
         _data = new string[size];
     }
+
+    public void Add(string input)
+    {
+        if (Capacity == Count)
+        {
+            Grow();
+        }
+
+        _data[Count++] = input;
+    }
+
+    private void Grow()
+    {
+        var newData = new string[Capacity * 2];
+        Array.Copy(_data, newData, Count);
+        _data = newData;
+    }
 }