Creating vector tiles for Valhalla

[glennon]

Since the previous valhalla docker workflow included the commands to process OSM pbf data into Valhalla-ready vector tiles, it would be useful to add a blurb in the doc or README about how that can be accomplished.

For example,

valhalla_build_admins -c conf/valhalla.json *.pbf
valhalla_build_tiles -c conf/valhalla.json *.pbf