added documentation to the introduction  for with, opaque types and more

Well some more text for the intro doc covering:
- with
- opaque types
- shared type aliases
- unsafely_as and as functions

It is mostly based on your e-mails on the mailing-list

I hope I can help you at least a bit to safe your time and make some of the the new and cool features available to the wider public.

Please read it because while I tried my best both in english and yeti - it is propably not enough

Thanks for your great work