doc: readme section for generated greybus manifest #47
Description
The Greybus Manifest Source file (.mnfs) and the Greybus Manifest Binary file (.mnfb) are both generated automatically from a board's DeviceTree overlay file (.dts).
Users might want to extract those build artefacts, so there should be some information in the README about where to find them (namely, at build/zephyr/greybus.mnfs and build/zephyr/greybus.mnfb, respectively).