Add warning on future breakage

Michal will update tex4ht ditribution... this will eventually make it to
texlive then to the CI server and it will break.  Adding his comment in
the code so we know.

book/tex4ebook.cfg

@@ -33,4 +33,13 @@
 }
 \makeatother
 
+% Warnig from Michal: Note that once you get the updated TeX Live with a new
+% `chemfig.4ht` file, you can remove this from the config file:
+%
+%```latex \pend:def\schemestart{\Picture+{}}
+%\append:def\CF_schemestop{\EndPicture} ```
+%
+%You could get an error otherwise, as there would be nested `\Picture+{}` and
+%`\EndPicture` commands.
+
 \EndPreamble