Rename Git "master" branch to "main"

[svengreb]

A recently started movement in the open source community with the aim of renaming the master branch to main resulted in an official change for many large and important projects including Git itself and GitHub. The github/renaming repository includes more details about this change, migration information and further steps as well as a roadmap for rolling this change out to the whole GitHub platform. A recent GitHub update already ensured that links to deleted branches are now redirected to the default branch of a repository.

In order to prepare this repository in advance the current master branch will be renamed to main and all references will be updated to match this change.

[svengreb]

Created the new main branch based on the latest master branch and updated the default branch settings of this repository. All references to the now deleted master branch are now redirected to the new main branch, e.g. this link to the repository README.