Add a description about replica_affinity option for cluster gem

[supercaracal]

closes #1067

I added a new feature to redis-cluster-client that we become able to specify the strategy for reading from replicas. I tried to simulate the environment of multiple availability zones with the tc command.

• https://github.com/redis-rb/redis-cluster-client/blob/dcfeb6f583043c22c7a8ee49f70156ffd6b95171/docker-compose.latency.yaml#L34-L51
• https://github.com/redis-rb/redis-cluster-client/blob/dcfeb6f583043c22c7a8ee49f70156ffd6b95171/.github/workflows/test.yaml#L261-L316

near node:

PING 172.18.0.5 (172.18.0.5) 56(84) bytes of data.
64 bytes from 172.18.0.5: icmp_seq=1 ttl=64 time=0.049 ms
64 bytes from 172.18.0.5: icmp_seq=2 ttl=64 time=0.052 ms
64 bytes from 172.18.0.5: icmp_seq=3 ttl=64 time=0.041 ms
64 bytes from 172.18.0.5: icmp_seq=4 ttl=64 time=0.039 ms
64 bytes from 172.18.0.5: icmp_seq=5 ttl=64 time=0.056 ms

far node:

PING 172.18.0.4 (172.18.0.4) 56(84) bytes of data.
64 bytes from 172.18.0.4: icmp_seq=1 ttl=64 time=2.13 ms
64 bytes from 172.18.0.4: icmp_seq=2 ttl=64 time=2.07 ms
64 bytes from 172.18.0.4: icmp_seq=3 ttl=64 time=2.07 ms
64 bytes from 172.18.0.4: icmp_seq=4 ttl=64 time=2.06 ms
64 bytes from 172.18.0.4: icmp_seq=5 ttl=64 time=2.15 ms

replica_affinity: :random

BenchCommand::ScaleReadRandom#bench_get = bench_get	 0.000531	 0.027782	 0.103212	 1.216746	12.433569

replica_affinity: :latency

BenchCommand::ScaleReadLatency#bench_get = bench_get	 0.000344	 0.001310	 0.008482	 0.086164	 0.762670