Add Section on using Tap-Spec or other Formatters/Reporters with Tap/Tape

[nelsonic]

@iteles this PR is my attempt to tidy up the issues in this repo and improve the readme for beginners:

• [x] add section on using tap-spec to format test output #3
• [x] section on using t.plan(2) for async assertions https://github.com/dwyl/learn-tape/issues/12
• [x] clarify creation of change-calculator.js file #19 @morkeltry's issue.
• [x] add npm init -y to installation command to ensure people have a package.json file. #26

Let me know if anything unclear. Thanks! 🌻

[codecov[bot]]

Codecov Report

Merging #41 into master will not change coverage. The diff coverage is n/a.

@@          Coverage Diff          @@
##           master    #41   +/-   ##
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  Coverage     100%   100%           
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  Files           2      2           
  Lines          28     28           
  Branches        2      2           
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  Hits           28     28

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