Add signup docs to README

[taion]

The docs for signups: https://auth0.com/docs/libraries/auth0js#sign-ups are not present in README, so they don't show up in GH or NPM docs. I found them eventually, but it might be nice if they were also documented on the README.

[taion]

Eh? This still isn't documented in the README: https://auth0.com/docs/libraries/auth0js#sign-up

[hzalaz]

We are trying to trim down the README and redirect most of the use cases/examples to the docs site, we just added a link to the docs to start and we are trying to figure out what content we leave in the README

[taion]

Got it, thanks.