Fixes for examples in readme.

[PatrykSzwer]

1. Example in Foundation -> Permissions section has wrong spacing:

public class ListInvoices: Permission
{
}

public class DisplayInvoice: Permission
{
    public Invoice Invoice {get; private set;}
    public DisplayInvoice(Invoice invoice)
{

this.Invoice = invoice;
}
}

2. I think that in example in check the rules section the new keyword before AuthorizationEnforcement is missing:

var enforcement = AuthorizationEnforcement(rules, new SystemUserAuthentication());

3. In prerequisite router section TODO part can be already replaced with link to Router description:

SecurityMiddleware is a feature that depends on Router. If you don't know what it is - go ahead and check it out // TODO LINK

cc: @joozek78