Information on the Hypothes.is plug-in from Scalar: "[With Hypothes.is] users are free to comment on selections of text and to set those comments to either public or private. In the public mode, all future visitors can view those comments in the Hypothes.is sidebar and reply to them if desired. While users must have a Hypothes.is account in order to make or reply to comments (to sign up for a Hypothes.is account, simply launch the Hyptothes.is sidebar and click Sign In > Create an Account) public comments are viewable to all visitors.
Hypothes.is thus allows per-paragraph, per-sentence and even per-word commenting for readers. It also allows groups of authors on a project to comment on and generate discussion threads around textual elements in a Scalar book. More information on the Hypothes.is project can be found at their website, here. To learn more about using the Hypothes.is plugin itself, click here."