Documentation of mason.cmake #18

Reminder to write documentation for how to use:

• Mason packages within cmake CMakelists.txt
• Specifically the variables and functions made available by mason.cmake

Current users of this methodology we can point to as examples are mapbox-gl-native, osrm-backend, and https://github.com/mapbox/rewrite-exit-destination-signage.

/cc @karenzshea