Writing documentation for your code/library/framework/project should be another one of these skills. I have written a lot of documentation for a project [1] recently, and it has made me realize the importance of this skill. It is often overlooked by programmers, myself included, as before this project, I never wrote proper documentation.
Sure, you can get technical writers to do it for you if you have a big team, but a lot of small, open source projects suffer from the lack of proper documentation.