improve README.md

• wording "You can also use enum."
• bad name var parsed = new CmdArgs();
• bad name var h = new PrintHelp
• inline var a1 = new Action1(); app.Add(a1);
• comment about not being mandatory static etc. public static MyType Parse(string arg)
• Then you have to stack up parsing layers sounds negative
• // add actions a bit more context
• Custom parsers add a note that custom parses have prio over build ins