[Introduction] Tell about the city/architecture metaphor for design spaces

[davelab6]

I propose that I will contribute more to the intro text along the lines of https://github.com/TypeNetwork/fb-Decovar/issues/3#issuecomment-279870867 - to provide more context for the problem which primary axes solve

[davelab6]

After visiting Mica this week, this should be done in a way that answers the concern that too much power in the hands of end users means they'll go to the "bad" locations.

Something about hand rails for the end user...