Update the README for Emacs 25 being buggy

As suggested in syl20bnr#4000, make clear that Emacs 25 is only partially
working.

README.md

@@ -104,8 +104,10 @@ If you prefer IRC, connect to the [Gitter Chat IRC server][] and join the
 
 ## Emacs
 
-Spacemacs is operational with Emacs 24.3 but Emacs 24.4 and above are
-recommended to enjoy the full experience.
+Spacemacs is operational with Emacs 24.3, but Emacs 24.4 and above are highly
+recommended to enjoy a full experience. The next Emacs major release, Emacs 25,
+is not *officially* supported but is partially working (i.e. bugs should be
+expected).
 
 Some modes require third-party tools that you'll have to install via your
 favorite package manager.