Add missing SPDX file

[tsteenbe]

Would be nice if the SPDX specification specifies itself with a SPDX file as CC-BY-3.0 AND MIT

[tsteenbe]

See related spdx/tools ticket for format to use spdx/tools#107

[goneall]

Still a good idea - but since this is non-breaking, moving this to 3.1