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 <[e-mail]>
Cc: David Woodhouse <[e-mail]>
Cc: Rob Landley <[e-mail]>
Signed-off-by: Yann Droneaud <[e-mail]>
Acked-by: Sam Ravnborg <[e-mail]>
Signed-off-by: Michal Marek <[e-mail]>

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