README.md: add fedora instructions #255

Job	Run time
Clang Tidy	7s
Build+test (stable, ubuntu-24.04)	3m 20s
Clippy nightly (optional)	31s
Clippy	32s
Build+test (stable, ubuntu-22.04)	1m 46s
Build+test (beta, ubuntu-24.04)	2m 18s
Check for documentation errors	42s
Format	12s
Build+test (beta, ubuntu-22.04)	1m 51s
Miri	1m 3s
Build+test (nightly, ubuntu-24.04)	2m 54s
Packaging	2m 58s
Build+test (nightly, ubuntu-22.04)	1m 47s
Produce release packages (fedora, 40, rpm)	2m 54s
Valgrind	2m 14s
Produce release packages (ubuntu, 24.04, deb)	2m 13s
	27m 22s

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