Add new episode on maintaining lessons

[raynamharris]

At a recent maintainer meeting, @hoytpr shared a "GitHub for Maintainer Dummies" set up tips for maintainers. I think it would be good to include some of these tips in this repo, perhaps as a new episode about lesson maintenance.

[brownsarahm]

I think this would be really nice especially for people who use the lesson for community lessons, not only official lessons.

[hoytpr]

Thanks @raynamharris and @brownsarahm , I'll gladly continue to clean it up and even put it in a lesson format as time permits.