Included Gitea Checks Plugin as an implementation in README.md

[crenan]

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Description:
As stated in Gitea Checks Plugin page, it implements the extension points defined by Checks API plugin. I just included that in README.md.

[codecov]

Codecov Report

Merging #179 (2474691) into master (74ecbc2) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #179   +/-   ##
=========================================
  Coverage     85.38%   85.38%           
  Complexity      152      152           
=========================================
  Files            17       17           
  Lines           746      746           
  Branches         57       57           
=========================================
  Hits            637      637           
  Misses           89       89           
  Partials         20       20           

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