docs: add how to use via nixpkgs #935

Job	Run time
Source code checks (stable)	34s
Benchmarks	11m 31s
Build (x86_64/linux)	1m 47s
Check for unneeded dependencies	1m 3s
Source code checks (1.75.0)	25s
Build (aarch64/macos)	2m 3s
Build (aarch64/linux)	2m 14s
Test (linux/x86_64)	1m 32s
Build (wasm32/wasi-0.2)	1m 7s
Test (macOS)	2m 1s
Build (x86_64/windows)	2m 7s
External tests / bash-completion test suite	8m 31s
OS target tests (Fedora/latest)	22s
OS target tests (Debian/testing)	16s
OS target tests (Arch Linux/latest)	27s
	36m 0s

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