Async code example in Readme

[idan-h]

    private async Task<XLWorkbook> BuildExcelFile(int id)
    {
        //Creating the workbook
        var t = Task.Run(() =>
        {
            var wb = new XLWorkbook();
            var ws = wb.AddWorksheet("Sheet1");
            ws.FirstCell().SetValue(id);

            return wb;
        });

        return await t;
    }

instead of

    private Task<XLWorkbook> BuildExcelFile(int id)
    {
        //Creating the workbook
        return Task.Run(() =>
        {
            var wb = new XLWorkbook();
            var ws = wb.AddWorksheet("Sheet1");
            ws.FirstCell().SetValue(id);

            return wb;
        });
    }

and could be even

    private XLWorkbook BuildExcelFile(int id)
    {
        //Creating the workbook
        var wb = new XLWorkbook();
        var ws = wb.AddWorksheet("Sheet1");
        ws.FirstCell().SetValue(id);

        return wb;
    }

(and then Task.Run(...) in the main function)