Add note about where where mystery objects come from
Add note about where where mystery objects come from

As @ljharb pointed out, we are using some functions like `css()` and
`withStyles()`, but make no mention of what they are or where they come
from. Since they come from react-with-styles, let's make this clear.

naustudio/javascript,bl00mber/javascript,nulogy/javascript,pokelondon/javascript,webcoding/javascript-style-guide,naustudio/javascript,LodoSoftware/javascript-style-guide,practicefusion/javascript,allanfish/javascript,LodoSoftware/javascript-style-guide,addshoppers/javascript,xalexec/javascript,cloudability/javascript-style-guide,airbnb/javascript,appcom-interactive/javascript,whichdigital/javascript-coding-standards,getjll/JavaScript-Style-Guide,ljharb/javascript