Hey! ReadMe.io founder here. We custom built ours, because we couldn't find anything else on the market that worked the way we wanted.

The closest I've found otherwise is ContendBuilder.js:

http://www.innovastudio.com/content-builder.aspx

SirTrevor is similar, but without drag+drop:

http://madebymany.github.io/sir-trevor-js/example.html

For us, it was built using two linked jQuery Sortable lists (the sidebar, and the actual content). It's easier than it sounds, especially with Angular :)