docs: add Valuer interface documentation and example (#1540)

Add documentation for the Valuer interface introduced in #1416.

  - add Valuer interface section and example snippet in doc.go
  - add a new runnable example at _examples/valuer/main.go
  - add Valuer Interface reference in README

  Closes #1538

## Fixes Or Enhances


**Make sure that you've checked the boxes below before you submit PR:**
- [x] Tests exist or have been written that cover this particular
change.

@go-playground/validator-maintainers