Update README.md

[edan-bainglass]

The README.md examples do not reflect the current state of the docs and suggest a different use of the work graph.

[superstar54]

@edan-bainglass suggests using task.calcfunction instead of asking users to import calcfunction from the aida.engine.

So update the README to

# define add calcfunction
@task.calcfunction
def add(x, y):
    return x + y

[edan-bainglass]

Indeed. The fact that AiiDA's mechanics are supported is a neat feature. But at the end of the day, the real gain from aiida-workgraph is that one doesn't need to know about AiiDA (e.g. no AiiDA imports necessary).