Make docs for required more explicit regarding boolean values

[alexongh]

required validates that the value is not the data types default zero value. For boolean the default zero value is false. Therefore, a false boolean value would be interpreted as not matching the required validation and fail.

This, in my opinion, is a little unexpected. Judging from the name required only, I expected that a false boolean value would be ok, since a value was provided.

Nevertheless, in my opinion, this behavior should be documented a little more explicitly. Which is why I propose this change to the docs.

• Tests exist or have been written that cover this particular change.
  (this is only a docs change)

@go-playground/validator-maintainers

[coveralls]

coverage: 74.243%. remained the same
when pulling 043e9ee on alexongh:master
into e20b948 on go-playground:master.