Policy for adding new files to OpenWrt

[Paul Spooren]

Hi team,

Based on PR#3427 on GitHub[0] I'd suggest we add a policy that every
added file needs a SPDX header. This likely simplifies merging patches
into mainline Kernel and probably makes some lawyers happy.

There are fancy tools to do so, like the `reuse` project[1].

Best,
Paul

[0]: https://github.com/openwrt/openwrt/pull/3427
[1]: https://github.com/fsfe/reuse-tool