Remind why GNUmakefile gets read

On (info "(make) Overriding Makefiles") we see
   If you say ‘make foo’, ‘make’ will find ‘GNUmakefile’, read it, and
see that to make ‘foo’, it needs to run the recipe ‘frobnicate > foo’.

OK, but mention (info "(make) Makefile Names") in that sentence too,
else users won't know that the name GNUmakefile is special.