Fixes typo in installation instructions

[fionnbharra]

Use yarn add rather than just yarn

[CLAassistant]

All committers have signed the CLA.

[codecov[bot]]

Codecov Report

Merging #100 into master will not change coverage. The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #100   +/-   ##
=====================================
  Coverage      94%    94%           
=====================================
  Files           8      8           
  Lines         584    584           
=====================================
  Hits          549    549           
  Misses         35     35

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