Added import using wildcard to docs

[thesunny]

As per #377

With respect to the documentation, I import the wildcard as be which is what my library was called but the reason is that it is fairly descriptive. e.g. be.optional(be.string()). It also doesn't conflict with other methods like is.

However, that's a personal choice. I can change it to something else like ss so we'd end up with ss.optional(ss.string()).

[ianstormtaylor]

Thank you @thesunny! I just changed it to s for now so that the docs aren't too opinionated on it, but you're right that folks can choose whatever name like best.