FollowingNotes is a Chrome extension that adds a 'notes' option to each person in your twitter.com/following list. Data is stored used the Chrome storage API.
[Chrome Web Store link] [1] [1]: https://chrome.google.com/webstore/detail/followingnotes/hfmgacfkginkihcehdhfhbkncmffmebm "Chrome Web Store link"