How to bind code behind variables in WPF

public partial class MainWindow : Window
{
    private const string HelloMessage = "Hello World";
    public string Hello { get { return HelloMessage; } }

    public MainWindow()
    {
    }
}

<Window>
    <Label Content="{Binding Path=Hello}"/>
</Window>

using System.ComponentModel;
using System.Runtime.CompilerServices;
public partial class MainWindow : Window, INotifyPropertyChanged
{
  public MainWindow()
  {
    InitializeComponent();
    DataContext = this;
  }
  private string _Hello = "Select Workflow Variant:";
  public string Hello
  { 
    get { return _Hello; }
    set
    { 
      _Hello = value;
      OnPropertyChanged();
    }
  }
  #region INotifyPropertyChanged Members

  public event PropertyChangedEventHandler PropertyChanged;

  /// <summary>
  /// Raises this object's PropertyChanged event.
  /// </summary>
  /// <param name="propertyName">The property that has a new value.</param>
  protected void OnPropertyChanged([CallerMemberName]string propertyName = null)
  {
    PropertyChangedEventHandler handler = this.PropertyChanged;
    if (handler != null)
    {
      var e = new PropertyChangedEventArgs(propertyName);
      handler(this, e);
    }
  }
  #endregion
}