Update project documentation for how to run unit tests

[Julian Norton]

We have working unit tests thanks to @Tardog. The wiki needs to be updated to help developers understand and use them

https://github.com/JulianNorton/weather-10kb/wiki

DARK_SKY_API_KEY=************ GOOGLE_API_KEY=************ ./node_modules/.bin/mocha

[Loc Nguyen]

Done.