hacking: update advice w.r.t. synced files

hacking: update advice w.r.t. synced files

* HACKING: Update and improve advices and explanations about files
in the automake repository that are now owned by automake, but
mirrored from other upstreams.  Also, don't list these files
explicitly, rather point the reader to the $(FETCHFILES) variable
in Makefile.am.
* Makefile.am (FETCHFILES): Don't state that "there should be a
lot more here", as this is not true anymore today.  Only 'COPYING'
must be synced by hand.