Add topic about liveness

[leobalter]

Ref #19

[mhofman]

Reference usually means the WeakRef itself. Referent is used in other places to designate the target of the WeakRef.

However I'm wondering if using either in the definition of liveness makes sense since when used as a weak collection key, the symbol or object is neither.

Regarding how to best review this change, would it be possible to add the current published definition as a first commit, and your proposed changes as a second commit to allow easier diffing?