add codecov to show coverage

[stephengroat]

No description provided.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@7588d7d). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master      #33   +/-   ##
=========================================
  Coverage          ?   99.21%           
=========================================
  Files             ?       10           
  Lines             ?      637           
  Branches          ?        0           
=========================================
  Hits              ?      632           
  Misses            ?        5           
  Partials          ?        0

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[liufengyun]

Thanks a lot @stephengroat !