binder warning

[jaygambetta]

Fixes #231 by putting in the readme a warning about the binder

[nonhermitian]

I personally do not like having the Binder route as the highlighted way to run the notebooks. I would prefer to say run the notebooks locally is the default, and in the end best way to go. The binder is really to get started quickly, but the need to regenerate your token [which itself is hidden under the 'advanced' tab in your account (maybe should be easier to get)] really is not ideal. Perhaps we have a Binder image that has fewer tutorials that only rely on Aer, and then say you need to run locally to get hardware access?

Anyway, just my thoughts, this PR looks good.

[quantumjim]

There could also be the route of setting up our own server for Binder or jupyterhub, to allow the easy access option without the security issues. I'll look into it for the future.

[nonhermitian]

I also brought that up a while ago and, at least at the time, it was deemed too much work.

[jaygambetta]

We can change order. @quantumjim don’t waste too much time as others are looking into this as we speak.