Mention default firmware wifi password "airrohrcfg"

[TomHodson]

I was just setting up my device today and was puzzled that the hotspot wifi network required a password. I eventually found the password here: https://nettigo.eu/products/sensor-community-kit-sds011-bme280-english-language-harness-cable-edition but I think it should be mentioned on the main guide page.