pkg/uroot: add note about default binary directory.
pkg/uroot: add note about default binary directory.

u-root/u-root,GanShun/u-root,hugelgupf/u-root,hugelgupf/u-root,GanShun/u-root,u-root/u-root,u-root/u-root,rminnich/u-root,hugelgupf/u-root,hugelgupf/u-root,rminnich/u-root,rminnich/u-root,GanShun/u-root,rminnich/u-root