Readme: Mention creating ngrok account for development

[meeech]

When I was following the getting started guide (slack), everything worked great with yarn dev and using the provided ngrok url.

However, after starting/stopping server a bit, the ngrok url provided stopped working.

It wasn't clear to me that I needed to go register on the ngrok site for an account. (did I miss that somewhere in the docs?) Once I did that, and added my auth to ~/.ngrok2/ngrok.yml everything worked again.

If it is welcome, I am happy to open a pr to add this bit of info to the readme.