Added notes about versioning OpenTimelineIO-Plugins

[reinecke]

Addresses a TO-DO item from #1386.

Documented the OpenTimelineIO-Plugins versioning strategy in CONTRIBUTING.md

[jhodges10]

Looks good!

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.71%. Comparing base (c0e97b0) to head (aaca1a2).
Report is 12 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1769      +/-   ##
==========================================
- Coverage   84.11%   81.71%   -2.41%     
==========================================
  Files         198      176      -22     
  Lines       22241    12315    -9926     
  Branches     4687     3028    -1659     
==========================================
- Hits        18709    10063    -8646     
+ Misses       2610     1711     -899     
+ Partials      922      541     -381     

Flag	Coverage Δ
py-unittests	81.71% <ø> (-2.41%)	⬇️

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see 66 files with indirect coverage changes

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