Document delegation of terminate()

[igorw]

As a follow-up to #10, since we have established that terminate() should always delegate, we should document this properly.

• The README of stack/builder should explain what the expected behaviour of terminable middlewares is, and what happens if one of the middlewares in the chain is not terminable.
• There should be some information on the stack website (maybe a STACK-n convention) explaining that middlewares implementing Terminable MUST delegate terminate(), always and unconditionally.