README: Move to "meson setup [options]"

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

Seen a few other repos do this like:
https://github.com/openbmc/phosphor-hwmon/commit/46a29b7f21c03fbe7230abc6feff3991efd9f76b



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

Change-Id: I3176fbf0df064c17473f5fc6ad723ca03ed0d04f
Signed-off-by: Gunnar Mills <gmills@us.ibm.com>