Additional example for FreeT in docs

Hi,
I have recently used FreeT and found that docs provide only pretty specific example with State monad.

Would you mind accepting docs PR with additional basic Future Option example?

After consulting on gitter I ended up with additional transformation for Option ~> OptionT, which I am not sure.

import cats.free._
import cats._
import cats.data._
import cats.implicits._
import scala.concurrent.ExecutionContext.Implicits.global
import scala.concurrent.Future

sealed trait Ctx[A]

case class Action(value: Int) extends Ctx[Int]

def op1: FreeT[Ctx, Option, Int] =
  FreeT.liftF[Ctx, Option, Int](Action(7))

def op2: FreeT[Ctx, Option, Int] =
  FreeT.liftT[Ctx, Option, Int](Some(4))

def op3: FreeT[Ctx, Option, Int] =
  FreeT.pure[Ctx, Option, Int](1)

val opComplete: FreeT[Ctx, Option, Int] =
  for {
    a <- op1
    b <- op2
    c <- op3
  } yield a + b + c


// ----interpreters----

type OptFut[A] = OptionT[Future, A]

def futureInterpreter: Ctx ~> OptFut = new (Ctx ~> OptFut) {
  def apply[A](fa: Ctx[A]): OptFut[A] = {
    fa match {
      case Action(value) => OptionT.liftF(Future(value))
    }
  }
}

def optFutLift: Option ~> OptFut = new (Option ~> OptFut) {
  def apply[A](fa: Option[A]): OptFut[A] = {
    fa match {
      case Some(value) =>
        OptionT(Future(Option(value)))
      case None =>
        OptionT.none
    }
  }
}

val hoisted = opComplete.hoist(optFutLift)
val evaluated = hoisted.foldMap(futureInterpreter)

val future = evaluated.value
// Some(12)

Start by locating the existing FreeT documentation and its State monad example, then review the proposed Future and Option example in the issue. Add a basic example covering the shown FreeT operations and interpreters, and confirm the documented result is correct and readable.