multiboot2-common: improve README and diagrams #1071

Job	Run time
build (nightly) / rust	25s
build (stable) / rust	40s
build (msrv) / rust	18s
build no_std (msrv) / rust	23s
style (msrv) / rust	23s
tests with miri (nightly) / rust	1m 34s
style (nightly) / rust	28s
build no_std (nightly) / rust	36s
build no_std (stable) [w/o builder] / rust	15s
style (stable) / rust	1m 5s
build no_std (stable) / rust	14s
build no_std (stable) [Windows] / rust	58s
	7m 19s

Provide feedback