Line Note is VSCode extension to add notes to the line of code.
Select lines and invoke Add note at current position from the command palette or context menu.
You can see the note you wrote as hover text.
Notes are saved in $PROJECT_ROOT/.vscode/linenote like .vscode/linenote/path/to/code.js#L13-L42.md.
Tips: You can put hyperlinks in notes by writing #L42 or ../foo.js#L12-L15.