Please document import for Scalatest 3.1

[cksnp]

Using Selenium doesn't have a reference to scalatestplus; after some try-and-error I realized I need an import like this:

libraryDependencies += "org.scalatestplus" %% "selenium-2-45" % "3.1.0.0"

It would be nice to reflect this change in Scalatest docs.