Add documentation for route_N attributes

[ZeLonewolf]

This is a documentation update for openmaptiles/openmaptiles#1152, which adds route_* attributes to the transportation_name layer, in order to expose the data needed to render highway shields for concurrent routes.

[TomPohys]

Thank you, documentation updated in https://github.com/openmaptiles/www.openmaptiles.org/pull/75

The documentation is generated by https://github.com/openmaptiles/www.openmaptiles.org/blob/master/generate-jekyll-layer-docs.sh from layer.yaml files in OMT

[ZeLonewolf]

@TomPohys just to clarify, if we update layer.yaml in OMT, there is no need to post PRs to this repo?

[TomPohys]

Yes, you are right, it is not needed. The openmaptiles.org/schema should represent the last stable release and not the state of the master branch.