What is Atom?
The atom editor is great for viewing and editing markdown documents offline. It's opensource and feels remarkably similar to Sublime Text, which is what I think they are going for.
The file extension for a markdown file is ".md" and you may have seen them on the README files in Github.
sudo apt-add-repository ppa:webupd8team/atom -y sudo apt-get update sudo apt-get install atom -y