Add code-cov coverage badge

[vladistan]

No description provided.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.92%. Comparing base (ec82c74) to head (44b79a5).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #26   +/-   ##
=======================================
  Coverage   98.92%   98.92%           
=======================================
  Files          19       19           
  Lines        1768     1768           
=======================================
  Hits         1749     1749           
  Misses         19       19           

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[vladistan]

@AdamFinkle @thadk this adds code-cov badge to the readme.

But you have to edit this PR:

1. Go here https://app.codecov.io/gh/codeforboston/greenbutton_objects/config/badge

2. Click on Badges and Graphs

3. Copy markdown code for the badge

4. Paste into the readme

5. Optionally to make Git history look nice, squash your change with this commit. Or you git commit --amend