Update changelog for 2.10.0 beta2 (dotnet#615)

Initial validation attribute support (dotnet#590)

This change removes the need for some of the data files by moving the validation info to be expressed as attributes on properties/classes.

Edit OpenXmlBasePart property comment (fixes dotnet#511) (dotnet#528) (…

…dotnet#531)

Documentation correction.

Updated the copyright per nuget package requirements

adding new package icon (dotnet#377)

* adding new package icon

* updating icon source file

Clean up README (dotnet#359)

- Fixed some content wording
- Fixed markdown lint errors & warnings
- Added table of contents

Merging assembly versioning fixes (dotnet#212)

* changed 2015 to 2017 for VS build instructions

* better version of VS build section

* .gitignore: added *.orig

* update readme for 2.7.2 tagging

Merge pull request dotnet#162 from twsouthwick/disable-symbols

Exclude symbols from signing and artifacts

Release of Open XML SDK Version 2.6.0