example: demonstrate the process of validating a signup request

[nodivbyzero]

Fixes Or Enhances

This PR adds a new example function to demonstrate the process of validating a signup request.

Make sure that you've checked the boxes below before you submit PR:

• Tests exist or have been written that cover this particular change.

@go-playground/validator-maintainers

[coveralls]

coverage: 74.318%. remained the same
when pulling af87c3f on nodivbyzero:example-simple-signup
into 6c3307e on go-playground:master.