Add section about Elm flags

[naazy]

Flags are really useful to pass data from JS to your Elm init function to produce the initial state. I think it would be useful to write up a small piece on flags with a simple example.

http://stackoverflow.com/documentation/elm/3771/the-elm-architecture#t=201702261950413123149

[gabrielperales]

I found this article talking about how to pass environment variables to elm with webpack and flags. I hope it can be useful.

[iteles]

Thanks @naazy ! This was added in the PR referenced above.