fix example, document requirements to run example

[philippkeller]

Hi @klokan
Thanks for providing this repo. It's so hard to get documentation how to get flask to work with firebase.
Still, I had some difficulty getting your example to work, I figured all out and would like to contribute my changes to your README.md file.

[philippkeller]

I also created a blog post covering this python module here

[keosak]

Hello @philippkeller,

This repository is pretty much dead. I suggest you make your own fork if you want to develop it. We won't be making any more changes.

[klokan]

I would love to merge this PR @keosak - is there any danger in doing so?

@philippkeller did a great job here. (greetings to Zurich)

[philippkeller]

@klokan yay, greetings back to Switzerland 😄

@keosak I would be ok with forking it, if you don't want to maintain. The code looks very strong (a bit above my python level). What I would do with it is to post it to PyPi, update the versions and add one or two more social login providers. Apart from these things the code is running flawlessly. Really nice project.