Workflow change

[5A11]

To simplify the development and contribution workflows, we have recently made the decision to stop using the develop branch for feature consolidation and also renamed the default master branch to main.

Therefore from now on, new branches should typically be off of the main branch, and pull requests should also be typically made against the main branch. This means that the main branch (instead of develop) will be where features are accumulated over time.

Before this change, those wanting the latest features referred to the develop branch and those wanting a stable code base referred to master. After this change, the latest features can now be found on the main branch. This has the side effect that main may not be as stable as it used to be at all times and those wanting stable code bases should now refer to release tags.

The Contribution Guides is updated with up-to-date instructions for how to contribute to the code base.