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 80.26%. Comparing base (408baed) to head (aeee73b).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #658   +/-   ##
=======================================
  Coverage   80.26%   80.26%           
=======================================
  Files          75       75           
  Lines        1140     1140           
=======================================
  Hits          915      915           
  Misses        225      225           

Flag	Coverage Δ
pull_request	80.26% <ø> (?)
push	80.26% <ø> (ø)

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