document imports

[stew]

We need a document which states exactly what needs to be import when, so that newcomers know exactly what imports do what, and what the basic set is they can use to get going.

[longcao]

For posterity: @non mentioned in the round table discussion at the Typelevel Summit unconference that it should be the standard to publish only:

import cats.implicits._

as the fully-capable gateway to using cats, and that more specific imports can be used but with the caveat of being a) not exhaustively documented and b) for more advanced usage. IMO, part of this would include rewriting current tut documentation to use only import cats.implicits._, and publishing that this is the recommended pathway.

(hope I remembered correctly!)

[lvicentesanchez]

+1 for using import cats.implicits._ in the documentation, but I would love to have an advanced usage section with an extended version of what @noelmarkham has started here.

[longcao]

@lvicentesanchez 👍 that's pretty nice! I think that cheat sheet might be good to have in the documentation, perhaps as a sub-bullet to core in "Project Structure" (http://typelevel.org/cats/)

[noelmarkham]

I've started a PR at #936

[ceedubs]

There has been an imports guide for a while. One thing that it doesn't include is a guide on piecemeal imports, so I've created #1668 to track that.