add npm badge

[Almenon]

so user can easily navigate to npm site

[codecov-io]

Codecov Report

Merging #36 into master will not change coverage. The diff coverage is n/a.

@@          Coverage Diff           @@
##           master     #36   +/-   ##
======================================
  Coverage    8.75%   8.75%           
======================================
  Files           2       2           
  Lines         274     274           
======================================
  Hits           24      24           
  Misses        250     250

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