Enhance warning when FAUST_SECRET_KEY is missing

[jasonbahl]

Currently when running npm run build without a FAUST_SECRET_KEY in my .env file, I get the following output:

> faust build

warn - Could not find FAUST_SECRET_KEY environment variable.
warn - Some functionality may be limited.

I would love to have more information on what functionality is limited. I think either some highlights on what the limitations are and/or a page on the website that documents the limitations and a link to that page would be very helpful.

That message doesn't provide me enough information to know if the limitations are critical to what I'm working on or something I can proceed without addressing immediately.