WiFiWeather Sample needs a readme with info on how to set it up

I went to go run the WiFi sample, but was getting an `unauthorized/401` error, and realized that i needed to go setup an account.

i think that it would be useful to have a readme.md in the root of the wifiweather project that had instructions on how to configure the project to get it to work, including:

 * specifying wifi credentials
 * setting up the weather API stuff