Add scalatestplus.* versions to 3.1.0 draft release notes for RC1

[bvenners]

Hi Chee Seng,

I think we should include version numbers for scalatestplus libraries in the ScalaTest release notes. Or at least point them to a page that has the latest, which could be the github page for the project. I'm not sure if you did the scalatestplus releases yet. Once those are out, please make a PR against the 3.1.0 release notes page that includes them.

Thanks.

Bill