README.md: Update documentation for using meson
README.md: Update documentation for using meson

The README was outdated and was pointing to using automake instead of
meson.

Change-Id: Id8dcf2cdb1a5546a56e77328342779e10cedea20
Signed-off-by: Brandon Kim <dbcb4743d083bf663ac612bcd5bbeafe818a4d70@google.com>

openbmc/stdplus,openbmc/stdplus