Adding CONTRIBUTING.md

[TWJW-SANGER]

Adding CONTRIBUTING.md to outline our process for accepting external contributions.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 33.69%. Comparing base (d3b14df) to head (913b424).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #26   +/-   ##
=======================================
  Coverage   33.69%   33.69%           
=======================================
  Files           3        3           
  Lines         184      184           
=======================================
  Hits           62       62           
  Misses        122      122           

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