2.8.0 notes added (#358)

Updated DocumentFormat.OpenXml.Package.props to include 2.8.0 release notes
Updated README.md to include the date of 2.8.0 release

Author: tomjebo

DocumentFormat.OpenXml.Package.props

@@ -16,9 +16,14 @@
     <Copyright>Copyright 2017 (c) Microsoft Corporation. All rights reserved.</Copyright>
     <PackageTags>openxml office</PackageTags>
     <PackageReleaseNotes><![CDATA[
-Version 2.7.3 : *In development*
-- Manual saving was fixed when autosave is false
+
+Version 2.8.0 : December 28, 2017
+- Added default runtime directive for better .NET Native support
+- Fixed exceptions thrown when errors are encountered while opening packages to be consistent across platforms
 - Fixed issue on Mono platforms using System.IO.Packaging NuGet package (Xamarin, etc) when creating a document
+- Fixed manual saving of a package when autosave is false
+- Fixed schema constraint data and standardized serialization across platforms
+- Upgraded to System.IO.Packaging 4.4.0 which fixes some consistency with .NET Framework in opening packages
 
 Version 2.7.2 : June 7, 2017
 - Fixed assembly version information for strong naming.

README.md

@@ -102,7 +102,7 @@ Once System.IO.Packaging on .NET Core has feature parity with WindowsBase (ie st
 Change Log
 ==========
 
-Version 2.8.0 : *In development*
+Version 2.8.0 : December 28, 2017
 - Added default runtime directive for better .NET Native support
 - Fixed exceptions thrown when errors are encountered while opening packages to be consistent across platforms
 - Fixed issue on Mono platforms using System.IO.Packaging NuGet package (Xamarin, etc) when creating a document