Adding new parent class example in README

[pdevito3]

This adds a new example to the docs as discussed in #162. Adapted it to match the example in there already. Let me know if you want any mods!

[pyce-lb]

I hope this finds you even though the PR is already merged!

Whilst I was trying to understand the benefits of SmartEnum I was struggling to understand this example. I was questioning my knowledge of nested classes and their visibility, but I think I figured out what confused me.

The code added in this PR does not compile.
I presume it was meant to look like the code below (and there was some residue left after copy-pasting code from #162), although I still can't fathom what it really tries to demonstrate.

public class Manager
{
    private EmployeeType _employeeType { get; set; }

    public Manager(EmployeeType employeeType) => _employeeType = employeeType;

    public string Type
    {
        get => _employeeType.Name;
        set
        {
            if (!EmployeeType.TryFromName(value, true, out var parsed))
            {
                throw new Exception($"Invalid manage type of '{value}'");
            }
            _employeeType = parsed;
        }
    }

    public decimal BonusSize => _employeeType.BonusSize;
}

P.S. I would also suggest a different phrase instead of "parent class" as it is usually associated with inheritance, but in this case Manager is not derived from anything, it just holds a reference to EmployeeType.