Add small docs about xlsxwriter in user guide (pandas-dev#27366)

Author: Dr-Irv

doc/source/user_guide/io.rst

@@ -3231,6 +3231,10 @@ The look and feel of Excel worksheets created from pandas can be modified using
 * ``float_format`` : Format string for floating point numbers (default ``None``).
 * ``freeze_panes`` : A tuple of two integers representing the bottommost row and rightmost column to freeze. Each of these parameters is one-based, so (1, 1) will freeze the first row and first column (default ``None``).
 
+Using the `Xlsxwriter`_ engine provides many options for controlling the
+format of an Excel worksheet created with the ``to_excel`` method.  Excellent examples can be found in the
+`Xlsxwriter`_ documentation here: https://xlsxwriter.readthedocs.io/working_with_pandas.html
+
 .. _io.ods:
 
 OpenDocument Spreadsheets