Add docs describing the process of writing a new schema

[WilliamJamieson]

This PR addresses a request to add some documentation on how to create a schema.

Checklist

• Schema changes discussed at RAD Review Board meeting
• Added entry in CHANGES.rst under the corresponding subsection
• Updated relevant roman_datamodels utilities and tests
• Passed romancal regression testing on Jenkins / PLWishMaster. Link: https://plwishmaster.stsci.edu:8081/job/RT/job/romancal/XXX/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.38%. Comparing base (0fb1239) to head (0ec386f).

Additional details and impacted files

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  Coverage   95.38%   95.38%           
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  Files           4        4           
  Lines         195      195           
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  Hits          186      186           
  Misses          9        9           

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[nden]

This is a very useful write up. I left a few minor comments inline.
In addition, perhaps there needs to be a mention that schemas in general use indentation with the convention being 2 spaces. I've seen this trip new contributors.

[nden]

Is it worth mentioning anyOf?

[WilliamJamieson]

Currently we are only using the oneOf combiner.