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

Add NuGet.config back

Cherry-pick dotnet#835 and dotnet#818 for v2.12.1 release (dotnet#851)

* Allow rewriting of all malformed URIs regardless of target value (dotnet#835)

* Fixed `OpenXmlElement.RawOuterXml` to properly set null values without throwing (dotnet#818)

Co-authored-by: Alex Kindle <[email protected]>
Co-authored-by: aidilumarov <[email protected]>

Update README.md