sec 5.6 - Inheritance - add a forward reference to roles

[hbaragar]

The first sentence of section 5.6 is:

Object Oriented Programming provides the concept of inheritance as one of the mech-
anisms to allow for code reuse.

which is fine, but my understanding is that roles are the preferred mechanism for code re-use in perl6. As such, I think that roles should be mentioned in this paragraph (with a forward reference to the Roles chapter). Furthermore, there should be a discussion (possibly in a side bar) of the differences between inheritance and roles, and when to use each.