Document how to do single iteration of IEnumerable

[jzabroski]

From reddit:

using (var spreadsheet = new Spreadsheet(filepath))
{
    var records = spreadsheet.ReadWorksheet<TestClass, TestClassMap>("Sheet1");

    using (var writerNegative = spreadsheet.CreateWorksheetWriter<TestClass, TestClassMap>("Negative"))
    using (var writerPositive = spreadsheet.CreateWorksheetWriter<TestClass, TestClassMap>("Positive"))
    {
        foreach (var record in records)
        {
            if (record.Amount < 0)
            {
                writerNegative.WriteRecord(record);
            }
            else
            {
                writerPositive.WriteRecord(record);
            }
        }
    }
}