new editor for the design draft

In IETF-61 we called for a volunteer to become an editor
for the design draft. (The current editor, Tero, indicated
that he's becoming too busy elsewhere to continue editing.)

I am glad to announce that Hannes Tschofenig has agreed to
be the new editor. Welcome Hannes, and thanks for taking
this task! I would also like to thank Tero for editing
the earlier versions.

--Jari