documentation: Kernel and RootFS example config
documentation: Kernel and RootFS example config

Added example configuration in the *Getting Started* part
of the README.MD and removed a trailing comma in the json of
the /machine-config example

Signed-off-by: Bogdan Ionita <e9b1610f538b592485c1edc27cac9a6ec03a3bb6@amazon.com>

firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker