For most people, it was first introduced a little over a year ago when the kickstarter went live.
When @brianfeister did some interviews he’s mentioned that it was something that has been worked on over the last 5 to 10 years. Slowly being developed with friends and in play to become what it is today.
From an introductory place, I would probably start with the Kickstarter, as that is when it was introduced to the mass public. In the kickstarter it mentions this:
After years of testing, we brought Open Legend to the public eye in February 2016
so I suppose that is a starting place.