<!-- Describe your changes clearly and use examples if possible. -->
Add a section about Custom Validators, and a sample to create a custom
validator for zip codes. The sample is from a code that @0xTim had been
provided in an issue.
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Add 'How to create custom validators' to docs.
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Co-authored-by: Tim Condon <0xTim@users.noreply.github.com>
Hadi Sharghi
晋先森