DOC: Describe 'orphaned' tag in CONTRIBUTING

[effigies]

Changes proposed in this pull request

• Update CONTRIBUTING.md to direct new contributors to the orphaned tag

[codecov-io]

Codecov Report

Merging #2481 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2481   +/-   ##
=======================================
  Coverage   66.66%   66.66%           
=======================================
  Files         328      328           
  Lines       42557    42557           
  Branches     5277     5277           
=======================================
  Hits        28369    28369           
  Misses      13507    13507           
  Partials      681      681

Flag	Coverage Δ
#smoketests	50.76% <ø> (ø)	⬆️
#unittests	63.98% <ø> (ø)	⬆️

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