docs(readme): Add code coverage badge on README

[tonywu1999]

Motivation and Context

We would like the code coverage badge to be front of display on the package README.

Changes

• Add code coverage badge to README

Testing

Verified badge is present and looks correct on README of remote branch.

Checklist Before Requesting a Review

• I have read the MSstats contributing guidelines
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.79%. Comparing base (e2a760c) to head (2309b71).

Additional details and impacted files

@@           Coverage Diff           @@
##            devel       #8   +/-   ##
=======================================
  Coverage   59.79%   59.79%           
=======================================
  Files           3        3           
  Lines          97       97           
=======================================
  Hits           58       58           
  Misses         39       39           

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