Add example of generating a cert chain #505

Job	Run time
build-windows	13m 16s
Clippy	1m 36s
build (macOS-latest, stable)	52s
Measure coverage	9m 0s
build (ubuntu-latest, stable)	1m 4s
Documentation (stable)	2m 16s
Format	27s
build (ubuntu-latest, beta)	48s
Documentation (beta)	2m 6s
build (ubuntu-latest, nightly)	1m 7s
Documentation (nightly)	1m 39s
build (ubuntu-latest, stable 7 months ago)	56s
Documentation (stable 7 months ago)	2m 14s
	37m 21s

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