So it does! Trying it now, don't yet know how comprehensive it is, but it seems ...

zokier · on July 20, 2017

There was a campaign when it was new feature that showed a popup hilighting the feature after update, like this:

http://winaero.com/blog/wp-content/uploads/2015/05/Firefox-3...

Considering that reader view has it's own dedicated button and all, I'm not sure how it possibly could be made more promenient without being annoying. The popup already was bit on the edge of being annoying.