Added note about archive builds.

Follow up from [#13](https://github.com/ReactiveCocoa/ReactiveCocoa/issues/13#issuecomment-13312180).

I wanted to explain the point of `$(inherited)`, but I couldn't phrase it clearly and concisely enough and adding it when it's not needed or is already there shouldn't have any adverse effect. People who know what it's for can decide whether they want it or not, people who don't can just copy and paste the whole thing and not get any errors because of overwritten settings and such.