overleaf upgrade notes

[Karl Berry]

Let me mention that Overleaf just posted a long blog entry about
upgrading from TL 2017 to 2018. 
https://www.overleaf.com/blog/tex-live-upgrade-september-2019

It includes detailed descriptions of many package changes that can bite
users, and lots of other hints. Thanks, Overleaf!

This is what our "known bug" list should look like, except unfortunately
we have no resources with which to make it happen, systematic testing
not being something that volunteers much want to do. Such is life.

Looking forward to their analogous 2018->2019 post :). --best, karl.