added doc/apt.conf.sample

Unfortunately apt configuration is not straightforward at all
regarding being overridden: one can't just provide sources.list
but needs a corresponding apt.conf along with it, and that apt.conf
must disable the SysV-style snippet directories to avoid interesting
side effects of all the things getting overlaid.

So it's not surprising that torabora has asked for an example...
(thanks go to boyarsh@ since I asked him for an example long ago)

doc/apt.conf.sample

@@ -0,0 +1,8 @@
+// these must be unset to avoid picking up
+// /etc/apt/{apt.conf.d,sources.list.d,}/
+Dir::Etc::main "/dev/null";
+Dir::Etc::parts "/var/empty";
+Dir::Etc::SourceParts "/var/empty";
+
+// this is what caused the whole trouble
+Dir::Etc::sourcelist "/home/mike/apt/sources.list";

doc/variables.txt

@@ -6,7 +6,7 @@
 - APTCONF
   + задаёт путь к требуемому apt.conf
   + значение: пусто (по умолчанию системный) либо строка
-  + см. ../image.in/Makefile
+  + см. ../image.in/Makefile, apt.conf.sample
 
 - ARCH
   + определяет целевую архитектуру образа