Add link to GitHub releases #676

Job	Run time
build-windows	13m 48s
build (macOS-latest, stable)	3m 50s
Validate external types appearing in public API	1m 37s
build (ubuntu-latest, stable)	7m 39s
Build rustls-cert-gen as a standalone package	25s
Measure coverage	7m 14s
build (ubuntu-latest, beta)	7m 45s
Format & clippy	6m 49s
build (ubuntu-latest, nightly)	7m 40s
Documentation (stable)	1m 9s
build (ubuntu-latest, stable 7 months ago)	7m 49s
Check semver compatibility	3m 11s
Documentation (nightly)	1m 16s
	1h 10m 12s

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