Add CI for code coverage and update README

[fxamacker]

Closes #221

Description

• add GitHub Actions (coverage.yml) to generate code coverage report and upload to Codecov
• update README to add badges for ci, linters, and codeql.
• update README to add API Reference section with link to online godoc
• update README to add Contributing section

Caveats

Coverage reported by Go is around 81%, while Codecov reports roughly 10% lower.

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• [x] Targeted PR against main branch
• [x] Linked to Github issue with discussion and accepted design OR link to spec that describes this work
• [x] Code follows the standards mentioned here
• [x] Updated relevant documentation
• [x] Re-reviewed Files changed in the Github PR explorer
• [x] Added appropriate labels

[codecov-commenter]

Codecov Report

:exclamation: No coverage uploaded for pull request base (main@e966fd4). Click here to learn what that means. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #222   +/-   ##
=======================================
  Coverage        ?   70.49%           
=======================================
  Files           ?       14           
  Lines           ?     7439           
  Branches        ?        0           
=======================================
  Hits            ?     5244           
  Misses          ?     1584           
  Partials        ?      611           

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