Add some help to understand 'validate workflow'

[jayoung]

in this section I'd love a few words to help me understand what to look for when I click 'Validate workflow' in the Shiny app.

I think all I need to know at this point in the course is that the output looks a bit complex (especially for people who don't use R), but if I see valid=TRUE (or something like that), then I can proceed, and if I don't, then the other output will help me troubleshoot my WDL file.

Is that pretty much what you look for when you validate a workflow? what's the key marker of a good WDL here - valid, and/or validWorkflow and/or isRunnableWorkflow?

[jayoung]

idea: tell the user to validate helloHostname.wdl and walk them through what to look for