Add reference to Open-Xml-PowerTools in the Open-Xml-Sdk README.md
Add reference to Open-Xml-PowerTools in the Open-Xml-Sdk README.md

OfficeDev/Open-XML-SDK,modulexcite/Open-XML-SDK,syediddi/Open-XML-SDK,ClareMSYanGit/Open-XML-SDK,tarunchopra/Open-XML-SDK,modulexcite/Open-XML-SDK,EricWhiteDev/Open-XML-SDK,EricWhiteDev/Open-XML-SDK,syediddi/Open-XML-SDK,tomjebo/Open-XML-SDK,ThomasBarnekow/Open-XML-SDK