Add status badge for Codecov.io in README.md

[johlju]

Pull Request (PR) description

• Add status badge for Codecov.io in README.md.

This Pull Request (PR) fixes the following issues

None

Task list

• [x] Added an entry to the change log under the Unreleased section of the file CHANGELOG.md. Entry should say what was changed and how that affects users (if applicable), and reference the issue being resolved (if applicable).
• [ ] Resource documentation added/updated in README.md.
• [ ] Resource parameter descriptions added/updated in README.md, schema.mof and comment-based help.
• [ ] Comment-based help added/updated.
• [ ] Localization strings added/updated in all localization files as appropriate.
• [ ] Examples appropriately added/updated.
• [ ] Unit tests added/updated. See DSC Community Testing Guidelines.
• [ ] Integration tests added/updated (where possible). See DSC Community Testing Guidelines.
• [ ] New/changed code adheres to DSC Community Style Guidelines.

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This change is 

[codecov[bot]]

Codecov Report

Merging #235 into master will not change coverage. The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #235   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           7      7           
  Lines         451    451           
=====================================
  Hits          451    451

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Continue to review full report at Codecov.

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