Add example queries to README or wiki

[phdoerfler]

For people like me who never used grafana nor prometheus before it should be helpful if you added some of your queries that you used for the dashboard screenshot.

For instance, here's two of mine that are part of this panel:

rate(factorio_item_production_output{name="high-tech-science-pack"}[2m])*100

(factorio_item_production_output{name="high-tech-science-pack"} - (factorio_item_production_output{name="high-tech-science-pack"} offset 1m)) * (60 / 36)

The latter query has this weird factor to compensate for my reduced UPS (approximately 36 UPS instead of 60). But this is another issue.

@afex I can file a PR with some of my queries if you want

[bradjshannon]

I would really really appreciate any examples you can provide... This is my first grafana experience.

[afex]

any example queries are welcome! if you can, provide some context on whether they are more useful for early or late-game factories.