Add list of baked in validations in README.md

[psampaz]

Fixes #509

@go-playground/admins

Todo list:

• Decide on groups
• Sort all tags alphabetically
• Update pull request template with checkbox for updating both README and doc.go in case of a new validation

[psampaz]

@deankarn have a look.

I've included the whole list of baked-in validations are seen here https://github.com/go-playground/validator/blob/master/baked_in.go#L65.

Please arrange the groups as you wish. I made an initial grouping just for the example.

[PrakharSrivastav]

Thank you for all the efforts to assemble all validations at a single place.