update documentation with examples to use Go native types

[alovak]

This module supports the native Go types in data structures. So, instead of *field.String, you can use either *string or string. However, the documentation does not reflect this. It would be great to fix it.

Example:

// define a struct for the authorization request
type AuthorizationRequest struct {
	MTI                  string `index:"0"`
	PrimaryAccountNumber string `index:"2"`
	ProcessingCode       string `index:"3"`
	AmountTransaction    int    `index:"4"`
	TransmissionDateTime string `index:"7"`
	STAN                 string `index:"11"`
	LocalTransactionTime string `index:"12"`
	LocalTransactionDate string `index:"13"`
	ExpirationDate       string `index:"14"`
	// ...
}

The implemented proposal with the details can be found here: #277
If it's needed, here you can find all PRs to see how and what was changed: https://github.com/moov-io/iso8583/issues?q=is%3Aclosed+milestone%3A%22Use+native+Go+types+to+get%2Fset+data+from+ISO+8583+messages%22

[muddi900]

Hello.

I could only find references in the README, which I have changed. The docs directory doesn't seem to have any other.

[alovak]

@muddi900 wow! Thank you for your help!

I've noticed that we also need to update statements like this:

data.TransmissionDateTime.Value() => data.TransmissionDateTime. As Value() method is on the *field.String and is not on the string.