Hey everyone - as mentioned by others, the best way to have your content reviewed is to upgrade to Articulate 360 and then you can upload your storyboard/prototype directly to Articulate Review and get your feedback/comments right there.
But - for those of you who are still working in SL2 and for who that is not an option, what I used to do in the past is create a word doc with a simple table with 2 columns, the left-hand column had a screenshot of each slide/layer and the right column was for the SME to leave notes or comments. Not exactly ideal, but it worked out well. The longest part was usually taking the screenshots and creating the document with the tables and screenshots.
Also - if the person storyboarding doesnt have access to Storyline, consider doing this exact same process in Powerpoint. Almost everyone has powerpoint and it's also slide-based so gives you the same feel, you can create your hyperklinks between slides, and you can also import your PPT directly into SL when you are ready to start developing.
Hope this helps! :)