Added to be consistent with `MutableList.Builder`.
It's useful (compared to just using `ImmutableSet.Builder`) because can do `builder().addAll(ImmutableSet.of("abc")).add("abc")` without it complaining of duplicates.

I'd be happy with doing `.build().toImmutable()` instead. Would like consistency with `MutableList.Builder` though, so if not adding this then deprecate the equivalent method in `MutableList.Builder`.