todonotes
provides a nice method to add notes in latex document in the margin.
However, it doesn't work properly with latex documents with muli-level \input
statements
(eg. inputing a tex in another tex file that was inputted in the main file).
A simple solution is to use the following package:
\usepackage{marginnote} % to add notes to margins
\usepackage{amssymb} % for special latex symbols
\usepackage{xcolors} % for more broader range of latex colors