chore: Add CONTRIBUTING guide

[irvinlim]

Closes #20.

[codecov]

Codecov Report

Merging #26 (5ee54aa) into main (a477b3f) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #26   +/-   ##
=======================================
  Coverage   38.40%   38.40%           
=======================================
  Files         154      154           
  Lines        7794     7794           
=======================================
  Hits         2993     2993           
  Misses       4662     4662           
  Partials      139      139           

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[phanhuyn]

LGTM