Scratchy is a Visual Studio extension that lets you create notes in a handy tool window which you can dock wherever you like into your layout.
You can get Scratchy from Manage Extensions in Visual Studio, or you can download it directly via the Visual Studio marketplace.
After installing the extension, open the window via View -> Other Windows -> Scratchy. The contents of the scratchpad are saved per solution when the solution closes. Don't put anything important in there obviously, but this at least means you can retain the notes per session so won't be lost if you need to restart your machine or VS or something.