How can I add a note to a Visual Studio bookmark or line?

Due to the fact the answer marked as accepted has been deprecated in VS 2015 onwards, I used this solution instead:

Bookmark a line of code using the key cord Ctrl+K Ctrl+K, then open the Bookmarks window (Ctrl+K Ctrl+W). In the Bookmarks window you can rename your bookmarks to whatever you please.

I have tested this in both VS 2015 and 2017 (Community).

IMPORTANT NOTE: Make sure to File->Save All (Ctrl+Shift+S) every once in a while so you don't lose the bookmarks you've been working on if VS or Windows decide to crash. I have fallen victim to this and it is painful.

Instead of enabling Edit and Continue then entering a TODO: into the source code, you can create a Task List Shortcut (key cord Ctrl + K, Ctrl + H).

This adds an entry to the Shortcuts section of the Task List window. From there you can change the description of the task to remind you why you created it.