add notes for plugin usage

[PepperYan]

Checklist

• npm test passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

Description of change

[codecov]

Codecov Report

Merging #20 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #20   +/-   ##
=======================================
  Coverage   96.75%   96.75%           
=======================================
  Files           5        5           
  Lines         185      185           
=======================================
  Hits          179      179           
  Misses          6        6

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