private async Task AddTextToFile(String textToSave)
{
    var appFolder = Windows.Storage.ApplicationData.Current.LocalFolder;
    var file = await appFolder.CreateFileAsync("exposure.txt", 
        Windows.Storage.CreationCollisionOption.OpenIfExists);
    await Windows.Storage.FileIO.AppendTextAsync(file, textToSave + Environment.NewLine);
    // Look in Output Window of Visual Studio for path to file
    System.Diagnostics.Debug.WriteLine(String.Format("File is located at {0}", file.Path.ToString()));
}

private async void Button_Click(object sender, RoutedEventArgs e)
{
    await AddTextToFile(String.Format("MinimumExposure {0}", DateTime.Now.Millisecond.ToString()));
}