update README with IO example

[uhtewihptihwue]

As suggested by the activity on this gist https://gist.github.com/silgon/0ba43e00e0749cdf4f8d244e67cd9d6a, json file IO is a frequently used basic feature of this library. I believe that making such information available up front would be valuable for new users of this library

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.63%. Comparing base (de16550) to head (70390d1).
⚠️ Report is 19 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #342   +/-   ##
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  Coverage   98.63%   98.63%           
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  Files           5        5           
  Lines         441      441           
=======================================
  Hits          435      435           
  Misses          6        6           

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[quinnj]

The README now includes an example like the one proposed here