Instructions for contributing to the EC-Earth4 code in a feature branch

As the title says. This is a first publishable version I think. There is no mention of git pull and git submodule is used once (to update unedited submodules). In the covered scenario, the feature branch is never synced with its upstream. Something we have to consider adding at some point, particularly if we want the developer to do it at least once just before MR.