Add What to do when someone generates modules using functions

[roberth]

Preview

• [ ] pick an image from https://unsplash.com/search/photos/pattern
• [ ] link sprint report to previous sprint
• [ ] proofread
• [ ] merge PR (will be picked up soon by @nixos_org Twitter automation!)
• [ ] verify link on https://cards-dev.twitter.com/validator
• [ ] publish on twitter/hercules_ci, nixos weekly

[roberth]

Needs a better title.