Make it clear that the app needs to declare `appEnv`

[EdwinGuzman]

The readme should include information regarding declaring and setting up the appEnv variable or else search won't work (see image below). Typically, the variable is declared in webpack and users should know for any new apps created or create a fallback in the SearchBox component.

CC @LucasCioffi

[LucasCioffi]

Thanks @EdwinGuzman. I want to ask you on Monday if these defaults in webpack.config.js didn't work when you ran it locally.