The release and promo teams are collecting release notes for apps included in the upcoming 18.04 release in https://notes.kde.org/p/applications_18.04.0_announcement

I took a first crack at adding a section for Spectacle, but this should get more eyes on it. We should also consider generating some screenshots that depict new features added in the 18.04 release and adding links to them into that document.

Once this is finalized, we could also add the information into Spectacle's AppStream file in a new <release> tag, which will result in all the notes showing up for users of KDE Discover and other software center apps.