Added additional overload of Add which includes out parameter

[extremeandy]

Please link to https://github.com/i3arnon/ConcurrentHashSet/issues/14

[i3arnon]

I expected the out parameter to only return the existing value if it was there and default otherwise. This is usually the pattern with a bool returning method with an out result (e.g. TryGetValue)

[extremeandy]

That makes sense for 'Try' type methods, but I think in the case where we are always dealing with a method that will succeed, the meaning of the out parameter is different, and that is OK.

Another option could be to have another method e.g. AddAndGet which returns a Tuple.