Add instruction for npm install with -D for development dependency

[yangli-io]

Issue:

What I did

Update docs to add -D for development dependency

How to test

[shilman]

Great catch! Thanks @yangli1990 !! 🏅

[codecov]

Codecov Report

Merging #1168 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1168   +/-   ##
=======================================
  Coverage   13.31%   13.31%           
=======================================
  Files         199      199           
  Lines        4589     4589           
  Branches      725      725           
=======================================
  Hits          611      611           
  Misses       3349     3349           
  Partials      629      629

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