Update README.md

[lhy1024]

What problem does this PR solve?

add key-type description

Release note

• No Release note

[codecov-commenter]

Codecov Report

Merging #2486 into master will increase coverage by 0.13%.
The diff coverage is 71.18%.

@@            Coverage Diff             @@
##           master    #2486      +/-   ##
==========================================
+ Coverage   77.01%   77.15%   +0.13%     
==========================================
  Files         205      205              
  Lines       21948    21994      +46     
==========================================
+ Hits        16903    16969      +66     
+ Misses       3755     3746       -9     
+ Partials     1290     1279      -11     

Impacted Files	Coverage Δ
server/cluster/coordinator.go	74.07% <7.14%> (-0.45%)	⬇️
server/cluster/cluster.go	80.91% <71.42%> (+0.62%)	⬆️
client/base_client.go	90.60% <100.00%> (+3.70%)	⬆️
client/client.go	71.10% <100.00%> (+1.50%)	⬆️
pkg/cache/ttl.go	90.66% <100.00%> (+4.72%)	⬆️
server/cluster/cluster_worker.go	72.95% <100.00%> (+0.22%)	⬆️
server/grpc_service.go	59.96% <100.00%> (+1.98%)	⬆️
server/schedulers/base_scheduler.go	60.86% <0.00%> (-8.70%)	⬇️
server/tso/tso.go	79.56% <0.00%> (-6.57%)	⬇️
pkg/etcdutil/etcdutil.go	78.26% <0.00%> (-5.80%)	⬇️
... and 14 more

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[lhy1024]

/merge

[sre-bot]

/run-all-tests

[sre-bot]

@lhy1024 merge failed.

[lhy1024]

/merge

[sre-bot]

/run-all-tests