Contribution Guide - add chapter about legal information and authors

[BirgitBoss]

For preparation to do more and more specification work in github itself (up to now published pdf of the specification was the master) we need to check

• which kind of license information needs to be part of the files themselves
• an authors file

Example: https://github.com/OpenManufacturingPlatform/sds-bamm-aspect-meta-model/blob/main/CONTRIBUTING.md#license-headers--licensing

[g1zzm0]

The license information can clearly be found in the Repository https://github.com/admin-shell-io/aas-specs/blob/master/README.md There is no need to bloat files unnecessarily.

[g1zzm0]

Due to the change to the CC-BY-4.0 license, can you elaborate what legal information are missing? Otherwise, please close the PR.