Update readme, highlighting the functionality in Open-Xml-PowerTools
Update readme, highlighting the functionality in Open-Xml-PowerTools

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