When it was announced that Apple would no longer be developing Aperture their Raw workflow software, there was consternation in the Aperture community.
Apple stated that they intend to bring out Photos for Mac, which will be a combined iPhoto/Aperture replacement, but as yet apart from a short demo in June at WWDC there has been little information about what that entails.
Shortly afterwards Adobe announced that they were working on an Aperture Importer for Lightroom (it also imports iPhoto libraries too), and that has now been released.
The process is fairly straightforward and is detailed on the Lightroom Journal blog post.
One thing to note is that you need Lightroom 5.6 and above. You can check your Lightroom version by going to Help->System Info inside Lightroom.
Hopefully this provides a simple solution to all Aperture users who are looking to migrate to Lightroom.