Document that you can set database credentials as sys env

[PietrH]

Lotte had not spotted this option, thus it needs to be better documented somewhere!

• Create vignette
• Stop storing credentials after prompting
• Only prompt when interactive?

[PietrH]

I think I should add a vignette similar to rgbif: https://docs.ropensci.org/rgbif/articles/gbif_credentials.html