* automake.texi (Optional): Document m4_include.

* automake.texi (Optional): Document m4_include.
(Invoking aclocal): Shake a bit to introduce the various uses
of aclocal and explain that aclocal will use m4_include for
local files.
(Extending aclocal): Show how to install installable macros
in $(datadir)/aclocal, and make it clearer that writing installable
macros is not the only way to extend aclocal.
(Local Macros, Future of aclocal): New sections.