Documentation: headers_install.txt is part of kbuild

'headers_install.txt' from Documentation/make/ is related to Kbuild
so it must be moved in Documentation/kbuild/ directory.

As Documentation/make/ directory has only one file, it will be removed
as a consequence of moving 'headers_install.txt'.

Cc: Kirill A. Shutemov <kirill@shutemov.name>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Rob Landley <rob@landley.net>
Signed-off-by: NYann Droneaud <ydroneaud@opteya.com>
Acked-by: NSam Ravnborg <sam@ravnborg.org>
Signed-off-by: NMichal Marek <mmarek@suse.cz>

Documentation/kbuild/00-INDEX

 00-INDEX
 	- this file: info on the kernel build process
+headers_install.txt
+	- how to export Linux headers for use by userspace
 kbuild.txt
 	- developer information on kbuild
 kconfig.txt