[docs] Update BUILDING.md instructions on adding files and folders.
[docs] Update BUILDING.md instructions on adding files and folders.

project-chip/connectedhomeip,nestlabs/connectedhomeip,nestlabs/connectedhomeip,nestlabs/connectedhomeip,nestlabs/connectedhomeip,project-chip/connectedhomeip,project-chip/connectedhomeip,nestlabs/connectedhomeip,project-chip/connectedhomeip,nestlabs/connectedhomeip,project-chip/connectedhomeip,project-chip/connectedhomeip,nestlabs/connectedhomeip