Update CHANGELOG.md

Move unshipped APIs over to shipped (dotnet#1102)

Merge unshipped APIs into shipped (dotnet#1058)

Update README.md (dotnet#1034)

Add OOXML Validator to the related tools list

Update CHANGELOG for 2.14.0-beta1

Update CHANGELOG.md

Update CHANGELOG.md

Update CHANGELOG for 2.13.0-beta2

Update CHANGELOG for 2.13.0-beta1

Remove build for .NET Core 1.x