Add "making test data files" section to contributing guide

[ericpre]

Closes #128.

Progress of the PR

• Explain how user can make good test data files and the rational,
• [n/a] update docstring (if appropriate),
• update contributing guide (if appropriate),
• add a changelog entry in the upcoming_changes folder (see upcoming_changes/README.rst),
• Check formatting of the changelog entry (and eventual user guide changes) in the docs/readthedocs.org:rosettasciio build of this PR (link in github checks)
• [n/a] add tests,
• ready for review.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.25%. Comparing base (531dbf8) to head (ca8bd82).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #233   +/-   ##
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  Coverage   86.25%   86.25%           
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  Files          82       82           
  Lines       10561    10561           
  Branches     2295     2295           
=======================================
  Hits         9109     9109           
  Misses        934      934           
  Partials      518      518           

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