Good idea, although I do update the readme regularly with each release. I’ll add the forum link in the readme of the next release.
Meanwhile, if you have downloaded a zip file of the plugin, the readme will be included. The last stable version is 0.9.1 at Release Classical Extras Release 0.9.1 · MetaTunes/picard-plugins · GitHub - For some reason the main plugin page has not been updated from v0.9 which has a slight bug (I have no control over those updates).
The forum thread is at Classical Extras plugin (for Picard 1.4.x), which is also the place to look for announcements of new releases.