Documentation: Include list of API key permissions in readme.

[MicahZoltu]

I want to give this action the minimum set of permissions possible to my Pinata account, but I don't know what those are. When I create a pinata API key I have a bunch of options and don't know what to select.

[popovoleksandr]

Will work on that - thanks for suggestions ;)

[adamgall]

@anantaramdas any update on documenting what permissions are needed for the various features of this action? 😁