first stab at RESEARCH.md

[drbenvincent]

• Closes Create developer-focused RESEARCH.md for CausalPy #562
• Adds RESEARCH.md (First-pass attempt. 100% sure changes will be required. But a decent starting point.)
• Adds reference to that in CONTRIBUTING.md

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📚 Documentation preview 📚: https://causalpy--567.org.readthedocs.build/en/567/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.50%. Comparing base (efde473) to head (7e27812).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #567   +/-   ##
=======================================
  Coverage   96.50%   96.50%           
=======================================
  Files          31       31           
  Lines        4173     4173           
=======================================
  Hits         4027     4027           
  Misses        146      146           

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