Document how to supply multi-value input for access_token_scopes

[Velociraptor]

Because GitHub Actions has a limitation that YAML arrays cannot be used as inputs (which is a wild choice on their part IMO), different actions each have their own strategy for accepting multi-value inputs. There doesn't seem to be a standard approach, for instance get-secretmanager-secrets parses secrets as a multi-line input with each secret on a new line, whereas this repo parses list inputs as a single-line comma-separated list.

I found it confusing that the documentation just says "List" and you have to dig into the code to find the parseCSV to tell how a multi-value input should be provided for this input.