Add signed-off-by policy for Contributions

[devin-petersohn]

We can follow the Linux Kernel contributions rules for signed-off-by:

https://www.kernel.org/doc/html/v4.17/process/submitting-patches.html#sign-your-work-the-developer-s-certificate-of-origin

By making a contribution to this project, I certify that:

a.) The contribution was created in whole or in part by me and I have the right to submit it under the open source license indicated in the file; or
b.) The contribution is based upon previous work that, to the best of my knowledge, is covered under an appropriate open source license and I have the right under that license to submit that work with modifications, whether created in whole or in part by me, under the same open source license (unless I am permitted to submit under a different license), as indicated in the file; or
c.) The contribution was provided directly to me by some other person who certified (a), (b) or (c) and I have not modified it.
d.) I understand and agree that this project and the contribution are public and that a record of the contribution (including all personal information I submit with it, including my sign-off) is maintained indefinitely and may be redistributed consistent with this project or the open source license(s) involved.

Then you just add a line saying:

Signed-off-by: Random J Developer <random@developer.example.org>
using your real name (sorry, no pseudonyms or anonymous contributions.)