Remind install gunicorn in generated config

[chadwcarlson]

User feedback, hoping to make assumed start command more clear and avoid a first 502 push:

[akalipetis]

I would prefer to have this as a warning message when running the ify command, in case the dependency does not exist.

That's a good quick measure though - let's add it to the Platform.sh templates as well¹ and we are good to go for now.

Footnotes

1. https://github.com/platformsh/platformify/blob/main/platformifier/templates/generic/.platform.app.yaml ↩