Document the MOVE Centreline simplification

I think main documentation of what it is should go in https://github.com/CityofToronto/bdit_data-sources/blob/master/gis/centreline/readme.md

And then add links to the short-term counts data dictionary https://github.com/CityofToronto/bdit_data-sources/blob/master/volumes/short_term_counting_program/README.md