README: Move to "meson setup [options]"

Fixes the warning:
WARNING: Running the setup command as `meson [options]` instead of
`meson setup [options]` is ambiguous and deprecated.

Tested: The following work:
meson setup build
ninja -C build

Change-Id: I4d45fab5eb25d9355f21a8da260ee52b5ed4ba18
Signed-off-by: Pavithra Barithaya <pavithra.b@ibm.com>