README.md: add fedora instructions #254

Job	Run time
Clang Tidy	13s
Clippy	30s
Clippy nightly (optional)	33s
Build+test (stable, ubuntu-24.04)	3m 1s
Check for documentation errors	40s
Build+test (stable, ubuntu-22.04)	1m 39s
Format	12s
Build+test (beta, ubuntu-24.04)	3m 6s
Miri	1m 2s
Packaging	3m 21s
Build+test (beta, ubuntu-22.04)	1m 45s
Build+test (nightly, ubuntu-24.04)	3m 3s
Produce release packages (fedora, 40, rpm)	3m 17s
Valgrind	2m 14s
Build+test (nightly, ubuntu-22.04)	1m 43s
Produce release packages (ubuntu, 24.04, deb)	2m 34s
	28m 53s

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