Adding margin notes to a document is more convenient and better noticable than comments between the lines. Latex provides a very easy way to add margin notes without changing the margins of your document.
You can add anything you like, e.g. a formula in the math environment, etc. And you don’t even need to use a package. Adding margin notes is a standard command in Latex, you just have to know it 🙂 .
There you go:
\marginpar{A note in the margins.}
Also consider my post on how to make page margins visible.