There is a todo package that can be used for this purpose. Simply add these lines to your document and you can leave comments with \todo commands.
\usepackage{geometry}
The above package will be used in most documents, so no need to add it if its already there
\usepackage[colorinlistoftodos]{todonotes}
by default it will be on right, to put it on left (if you don’t have room beacuse of margins) use
\reversemarginpar
For commenting
\todo{this is an example comment}
Comments will be show as follows: