Explain how to customize a serializer

[gforcada]

Part of #1975

• I signed and returned the Plone Contributor Agreement, and received and accepted an invitation to join a team in the Plone GitHub organization.
• I verified there aren't any other open pull requests for the same change.
• I followed the guidelines in Contributing to Plone.
• I successfully ran code quality checks on my changes locally.
• I successfully ran tests on my changes locally.
• If needed, I added new tests for my changes.
• If needed, I added documentation for my changes.
• I included a change log entry in my commits.

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If your pull request closes an open issue, include the exact text below, immediately followed by the issue number. When your pull request gets merged, then that issue will close automatically.

Closes #

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📚 Documentation preview 📚: https://plonerestapi--1976.org.readthedocs.build/

[mister-roboto]

@gforcada thanks for creating this Pull Request and helping to improve Plone!

TL;DR: Finish pushing changes, pass all other checks, then paste a comment:

@jenkins-plone-org please run jobs

To ensure that these changes do not break other parts of Plone, the Plone test suite matrix needs to pass, but it takes 30-60 min. Other CI checks are usually much faster and the Plone Jenkins resources are limited, so when done pushing changes and all other checks pass either start all Jenkins PR jobs yourself, or simply add the comment above in this PR to start all the jobs automatically.

Happy hacking!

[davisagli]

@gforcada Looks okay, but please see my comment #1975 (comment), and it needs a changelog

@stevepiercy do you want to edit anything?

[gforcada]

@davisagli thanks for the review. I added the deserialization part, with a warning on the serialization about it. Also added the changelog entry :)

[stevepiercy]

From reading this page, I have no clue where to place the XML or the Python code. Would someone please add that information?

Also are there any missing steps? Assume that the reader doesn't have decades of experience with Plone.