Update Contribution Guideline w.r.t. approval

[BirgitBoss]

Major changes must first be reviewed and approved by the joint working group "Asset Administration Shell" of the [Platform Industrie 4.0](http://www.plattform-i40.de/) and [IDTA](https://industrialdigitaltwin.org/).

needs to be changed to

Major changes must first be reviewed and approved by the working stream "Specifications of the Asset Administration Shell" of the Open Technology Working Group of the [IDTA](https://industrialdigitaltwin.org/).