Add references to JSON metadata format

[alexjordan]

Github action tested here: https://github.com/alexjordan/xshady/actions

@jensdietrich to enable the action I had to change repo settings (in my fork) under Actions/General to

• "Allow actions created by Github"
• Allow this specific action: cue-lang/setup-cue@*

[jensdietrich]

@alexjordan I did change permissions for actions as requested.