ScanCode io ROADMAP

[pombredanne]

This is a placeholder to put a proper roadmap here
We have some elements there in the original SCIO roadmap at https://github.com/nexB/scancode-toolkit/blob/develop/ROADMAP.rst
And the goal of this issue is to create a ROADMAP.rst here
See also for the related aboutcode-org/scancode-toolkit#2919