util/kconfig/README.md: Add notes about adding a new quilt patch

The patches for kconfig need to be in a format compatible with the quilt
tool, and usually also contain a header with some additional info like
the git commit. This header is in the same format as patches produced by
`git format-patch`, but the diff style git uses is incompatible with
quilt and there does not seem to be a straightforward way to format the
diff section to work.

Add some documentation for a method I found to go from a git commit to a
quilt compatible patch with git headers.

Change-Id: I7a8bbe41e0864be1d28116742b6b8b3fc440cc31
Signed-off-by: Nicholas Chin <nic.c3.14@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/69458
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

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