der: linkify ITU X.690 in README.md (#1728) #2354

Job	Run time
build (1.81.0, wasm32-unknown-unknown)	56s
build (1.81.0, thumbv7em-none-eabi)	57s
test (1.81.0)	2m 30s
build (stable, thumbv7em-none-eabi)	54s
fuzz	2m 10s
build (stable, wasm32-unknown-unknown)	54s
test (stable)	2m 26s
minimal-versions	0s
	10m 47s

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