Add some documentation about how to make the field required

[grantholle]

It was confusing for me for a little while as to why I couldn't just use the required or min:1 rule for making a SelectPlus field required. I finally figured it out and created a small snippet.

I was going to actually contribute the rule itself with more options, but it's using private packagist so I can't require the illuminate/contracts package to create the rule.