CONTRIBUTING.md: embed the DCO

CONTRIBUTING.md: embed the DCO

In the future, we might want to use the "reuse" tool to check/track
licensing information of the repo. For better or worse it will complain
that the in-tree DCO.txt is missing SPDX copyright/licensing
information.

It can be worked around, by adding it into a separate file. Although a
much simpler solution is to just inline it.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Lucas De Marchi <demarchi@kernel.org>
Link: https://github.com/kmod-project/kmod/pull/pull/417
Signed-off-by: Lucas De Marchi <demarchi@kernel.org>