update readme for react apps

[mrdougwright]

This tripped me up more than once, so I thought I'd suggest updating the readme. Thanks for your time :)

[coveralls]

Coverage remained the same at 100.0% when pulling 0e2f4e6 on mrdougwright:react-readme-update into 825c1b2 on motdotla:master.

[maxbeatty]

this is an implementation detail of react and I worry that by this example you're sending sensitive information like database connection details to the client 🙁

furthermore, I don't think dotenv should be encouraged for client use. other build tools like webpack includes plugins that do the same thing