Update Contribution Guide -add chapter about release process

[BirgitBoss]

For preparation to do more and more specification in github itself (up to now published pdf of the specification was the master) we need to have a clearly defined release process including versioning schema. This issue is strongly related to the approval process of issue #9

Proposal similar to (In this case the working group is the joint working group "Asset Administration Shell" of IDTA and Plattform Industrie 4.0): https://github.com/OpenManufacturingPlatform/sds-bamm-aspect-meta-model/blob/main/CONTRIBUTING.md#release-process

[g1zzm0]

The process is well-defined in section https://github.com/admin-shell-io/aas-specs/blob/master/CONTRIBUTING.md#before-the-pull-requests . We use feature branches.

[BirgitBoss]

This is about releases and not about branch strategy but of course they are related. This is not yet well-defined.