Add codecov badge to README

[ianthomas23]

Add codecov badge to README so that it looks like this:

If you click on the badge it takes you to the codecov page so that you can look at the details.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.83%. Comparing base (e0f7ab9) to head (9c91bec).
⚠️ Report is 43 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #117   +/-   ##
=======================================
  Coverage   86.83%   86.83%           
=======================================
  Files          60       60           
  Lines        2340     2340           
  Branches      275      275           
=======================================
  Hits         2032     2032           
  Misses        308      308           

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