Update README.md

Author: Slamdunk

README.md

@@ -7,10 +7,15 @@
 
 This package is _NOT_ intended to be complete and flexible, but to be *fast*.
 
-[PHPOffice/PHPExcel](https://github.com/PHPOffice/PHPExcel) and [PHPOffice/PhpSpreadsheet](https://github.com/PHPOffice/PhpSpreadsheet) are great libraries,
+~[PHPOffice/PHPExcel](https://github.com/PHPOffice/PHPExcel) and [PHPOffice/PhpSpreadsheet](https://github.com/PHPOffice/PhpSpreadsheet) are great libraries,
 but abstract everything in memory before writing to the disk. This is
 extremely inefficent and slow if you need to write a giant XLS with thousands
-rows and hundreds columns.
+rows and hundreds columns.~
+
+PhpSpreadsheet now implements [memory saving techniques](https://phpspreadsheet.readthedocs.io/en/latest/topics/memory_saving/)
+and relying on XLS files is not necessary anymore.
+Please refer to the new https://github.com/Slamdunk/php-spreadsheet-helper for using this helper
+on the newer XLSX format.
 
 Based on [Spreadsheet_Excel_Writer v0.9.3](http://pear.php.net/package/Spreadsheet_Excel_Writer),
 which can be found active on [Github](https://github.com/pear/Spreadsheet_Excel_Writer).