Update README.md to inform about the Office2016 branch
Update README.md to inform about the Office2016 branch

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