Depends on your expected rate of growth, and how much time you expect each developer to waste in bootstrapping and mentoring. When you consider getting your third developer, then is the time to start thinking about documentation processes (as in managed and computer-accessible, not Z or UML).
Having been the second in a few startups, I can tell you that not having any sort of documentation is annoying. Do not expect anyone to read your code and understand how your mind works: if you don't document, you will have to sacrifice a significant amount of time training the new employee. You'll have to hire during a lull, and in a startup there's never a lull. As usual, Fred Brooks said it first.
Finally, Twitter is a good example of what acting first then designing second can achieve: unfixable scaling issues. Don't do that.
Having been the second in a few startups, I can tell you that not having any sort of documentation is annoying. Do not expect anyone to read your code and understand how your mind works: if you don't document, you will have to sacrifice a significant amount of time training the new employee. You'll have to hire during a lull, and in a startup there's never a lull. As usual, Fred Brooks said it first.
Finally, Twitter is a good example of what acting first then designing second can achieve: unfixable scaling issues. Don't do that.