Add pubsub to README

Fixes #258 
@silvolu is there anyone we should loop in for terminology/phrasing things like this?

> Okay I'm confused because the whole library is Alpha and the API is Preview.

Yes, but beyond the stability of our API, they should know the stability of the API they're accessing through us is even more likely to produce unexpected behavior. The "(Alpha)" note in the heading just makes it clear this api is not to be trusted.

> Also, do you want the "more prominent" alpha note quoted like you did in the comment?

Do you mean when I said `Edited "(Preview)" to "(Alpha)"`? If so, the quotes there were just to delimit the snippet of my comment that I changed. I don't think we need any quotes.