Documentation: Add code_development_model.md to soc/intel/index.md

Fixes Sphinx WARNING document isn't included in any toctree

Change-Id: I956ed23d87c7cbd65383cc64a6af7161e90d6611
Signed-off-by: Tom Hiller <thrilleratplay@gmail.com>
Reviewed-on: https://review.coreboot.org/27593
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
1 file changed