doc: add index to README.md #527

Job	Run time
[rust] rustfmt	3s
[rust] clippy	8s
[bash] lint	7s
spell-check	4s
[rust] test pinned	31s
[rust] test stable	22s
[rust] test beta	30s
[rust] test nightly	21s
[rust] test stable-musl	44s
[rust] test stable-x86	42s
[rust] test stable-aarch64	40s
[rust] test stable-arm-gnueabihf	44s
[rust] test stable-arm-musleabihf	49s
[rust] test stable-arm-musleabi	56s
[rust] test stable-powerpc64	45s
[rust] test stable-s390x	43s
[rust] test macos	26s
[rust] test win-msvc	2m 14s
[rust] test win-gnu	2m 22s
	13m 11s

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