README: Update the usage of setup script

The usage of setup script in README is missing the arguments, add them
so it is more clear how the script is called.

Before the script, the default build directory is `build`, so update the
example of romulus build to specify the `build` directory.

Otherwise, each machine has its own build directory. They do not share
the downloads dir or the sstate dir.

Signed-off-by: Lei YU <yulei.sh@bytedance.com>
Change-Id: I21fcb1a133941fbbff80a33a250ece687622477b