Update BUILDING.md to Include Content about Maintaining the Build System#67

Merged

woody-apple merged 1 commit intoproject-chip:masterfrom gerickson:user/gerickson/update-building

Mar 18, 2020

+36-1

Commits on Mar 17, 2020

Added additional content about maintaining the build system. Removed vestigal reference to the primordial project build system.
Grant Erickson
committed