Update README.md to inform about the Office2016 branch

Author: EricWhiteDev

README.md

@@ -3,6 +3,16 @@ Open-XML-SDK
 
 Release Version 2.6.1
 
+    News: The Office2016 branch on this repo contains a preview of the Open-Xml-Sdk v3.0, which
+	supports the Office 2016 extensions in the file format.  The biggest change to the markup was
+	the introduction of the ExtendedChartPart, which support the new chart types, such as Waterfall,
+	Box and Whisker, and Sunburst.  You can specify FileFormatVersions.Office2016 to validate this
+	markup.  This version of the SDK also supports using the strongly-typed OM to access the markup
+	in the new parts.
+	
+	We will merge the Office2016 branch into vNext at some point in the future, after receiving
+	feedback.
+
 The Open XML SDK provides open-source libraries for working with Open XML
 Documents (DOCX, XLSX, and PPTX).  It supports scenarios such as:
 - High-performance generation of word-processing documents, spreadsheets, and presentations.