Missing docs for creating classes with Metamodel::ClassHOW

[lefth]

Timo's ADT module gives an example of creating a type with Metamodel::ClassHOW, then adding constructors and methods:

https://github.com/timo/ADT/blob/master/lib/ADT.pm6

The most important method new_type, isn't even listed in our docs. It would be nice to have official documentation/examples. On the other hand, if this isn't a stable class and isn't supposed to be used, the docs should say so.