Doc: make README shorter #2861

Job	Run time
cluster-benchmark (cluster_benchmark)	46s
examples (stable, raft-kv-memstore)	2m 15s
external-stores (stores/rocksstore-v2)	5m 58s
examples (stable, raft-kv-rocksdb)	7m 47s
lint	6m 53s
examples (nightly, raft-kv-memstore)	2m 7s
openraft-feature-test (nightly)	48s
examples (nightly, raft-kv-rocksdb)	11m 44s
openraft-test (stable, 0)	2m 55s
openraft-test-bench (nightly)	1m 7s
openraft-test (nightly, 30)	3m 22s
openraft-feature-test (nightly, serde)	1m 29s
openraft-test (nightly, 0, single-term-leader)	3m 2s
stores (memstore)	44s
openraft-feature-test (nightly, single-term-leader)	53s
Build (nightly, bench,serde,bt,singlethreaded)	50s
stores (rocksstore)	6m 0s
openraft-feature-test (nightly, single-term-leader,serde,singlethreaded)	51s
tests-feature-test (nightly)	2m 38s
stores (rocksstore-compat07)	6m 0s
tests-feature-test (nightly, single-term-leader)	2m 41s
stores (sledstore)	54s
tests-feature-test (nightly, loosen-follower-log-revert)	2m 38s
	1h 14m 22s

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