Switch default branch from deprecated language

[anEXPer]

I'd like to change from a master branch to a develop branch.

This entails updating the README with a note about the change and instructions for changing remotes, changing the default branch in github, fixing the branch references in CI systems associated with this project (including NORSK), changing outstanding PRs to target the new branch, and deleting the old branch ref to avoid confusion.

I'm willing to undertake this work personally, but want to put the issue up here for discussion for a bit before setting out to do it, in case I've missed anything important.