Doc: Update README: new 0.8 releaase: 0.8.8 #2860

Job	Run time
Build (nightly, bench,serde,bt,singlethreaded)	47s
cluster-benchmark (cluster_benchmark)	46s
examples (stable, raft-kv-memstore)	2m 16s
external-stores (stores/rocksstore-v2)	6m 3s
examples (stable, raft-kv-rocksdb)	8m 7s
lint	6m 33s
examples (nightly, raft-kv-memstore)	2m 9s
openraft-feature-test (nightly)	47s
examples (nightly, raft-kv-rocksdb)	11m 41s
openraft-feature-test (nightly, serde)	51s
openraft-test (stable, 0)	2m 59s
openraft-feature-test (nightly, single-term-leader)	50s
openraft-test (nightly, 30)	3m 10s
openraft-test-bench (nightly)	54s
openraft-feature-test (nightly, single-term-leader,serde,singlethreaded)	50s
openraft-test (nightly, 0, single-term-leader)	2m 55s
stores (memstore)	50s
tests-feature-test (nightly)	2m 39s
stores (rocksstore)	6m 4s
tests-feature-test (nightly, single-term-leader)	2m 43s
stores (rocksstore-compat07)	6m 2s
tests-feature-test (nightly, loosen-follower-log-revert)	2m 40s
stores (sledstore)	52s
	1h 13m 28s

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