Documentation: Encourage documentation with code changes

Documentation: Encourage documentation with code changes

The resource allocator woes post-4.12 release showed room for
improvement on both discussion and documentation.  To encourage this
(and encourage reviewers to look out for issues in that space),
extend the review guidelines so that they encourage to more clearly
document the reason for a change with the change (commit message or
our documentation) and also to loop in the mailing list.

Change-Id: I1962dba3fe7e1a01fa4c8b0058297c7d050cb7b7
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/41493
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>