It's easier for readers and maintainers to have one doc instead of two. Once you determine that your project isn't redundant, then you should survey existing material with an eye to incorporating as much of it as possible into your project. The more content you can harvest from existing writing, the better. You may be able to not only save yourself from writing, but you may gain collaborators if you can get the support of How-To authors and others who have written material that is related to your project. An outline is a hierarchical breakdown of a topic that describes the structure of the artifact.
Outlines are the most important organizational tool for all actors involved in documentation.
Authors use an outline to guide the content as it is written. Editors use an outline to evaluate the topic, scope, quality, and progress of an artifact. Maintainers use an outline to fit revised material smoothly into an existing artifact, and, ultimately, Readers use an outline to navigate through documentation. An outline consists of a sequence of top-level, key ideas that encompass the scope of your topic. Each top-level key idea should be broken down into a second level of more specific details of each key idea.