Add dev command

[georgiana-b]

This PR adds a command to run the app in development with:

npm run dev

or

yarn dev

I also documented it in the README.

Addresses #46 although not fully since it doesn't update the guide.

[georgiana-b]

Disclaimer: I only tested it with npm run dev.

@anuveyatsu If you use yarn please check that yarn dev works too. If it doesn't I can remove it from the README.

[cbenz]

Oh, I almost submitted the same PR ;-)

It worked for me with npm run dev, yarn dev and yarn run dev.

[anuveyatsu]

@georgiana-b LGTM 👍