functional test: Update functional README.md

[wenjiaswe]

Please read https://github.com/etcd-io/etcd/blob/master/CONTRIBUTING.md#contribution-flow.

[codecov-io]

Codecov Report

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@@            Coverage Diff            @@
##             master   #10985   +/-   ##
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  Coverage          ?   64.39%           
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  Files             ?      401           
  Lines             ?    37551           
  Branches          ?        0           
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  Hits              ?    24181           
  Misses            ?    11761           
  Partials          ?     1609

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[gyuho]

@wenjiaswe Can we backport this to 3.4 branch as well? Thanks!