Add PHPSpreadsheet (successor of PHPexcel) to Office section.

Also mark PHPexcel as unmaintained.

README.md

 *Libraries for working with office suite documents.*
 
 * [ExcelAnt](https://github.com/Wisembly/ExcelAnt) - A library for manipulating Microsoft Excel documents.
-* [PHPExcel](https://github.com/PHPOffice/PHPExcel) - A library for working with Microsoft Excel documents.
+* [PHPExcel](https://github.com/PHPOffice/PHPExcel) - A library for working with Microsoft Excel documents. *unmaintained* 
 * [PHPPowerPoint](https://github.com/PHPOffice/PHPPresentation) - A library for working with Microsoft PowerPoint Presentations.
 * [PHPWord](https://github.com/PHPOffice/PHPWord) - A library for working with Microsoft Word documents.
+* [PHPSpreadsheet](https://github.com/PHPOffice/PhpSpreadsheet) - A pure PHP library for reading and writing spreadsheet files (successor of PHPExcel)
 
 ## Database
 *Libraries for interacting with databases using object-relational mapping (ORM) or datamapping techniques.*