What's the best source of emacs release notes - especially for reading prior to installing an update?
I searched Google for "emacs release notes". There are some posts about specific releases (and some, like this about Emacs 25.1, are really good), but no definitive source.
There's another question on this site which discusses what's new in Emacs 25. The answers and comments there point to some source locations - are those generalizable?