Add CI shield for opam package to README (#1972) #919

Job	Run time
install (dev, Ubuntu, ubuntu-latest, 4.09.1)	14s
install (dev, macOS, macos-latest, 4.14.2)	31m 3s
install (dev, Windows, windows-latest, 4.13.1, -async-proofs-j 1, git+https://github.com/ocaml/oc...	1h 27m 32s
install (8.19.0, Ubuntu, ubuntu-latest, 4.09.1)	8s
install (8.19.0, macOS, macos-latest, 4.14.2)	38m 13s
install (8.19.0, Windows, windows-latest, 4.13.1, -async-proofs-j 1, git+https://github.com/ocaml...	1h 31m 27s
install (8.18.0, Ubuntu, ubuntu-latest, 4.09.1)	10s
install (8.18.0, macOS, macos-latest, 4.14.2)	33m 23s
install (8.18.0, Windows, windows-latest, 4.13.1, -async-proofs-j 1, git+https://github.com/ocaml...	1h 36m 57s
	6h 19m 7s

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