Add installation instructions for Atom

[k8hertweck]

Given current issues with the SWC Windows installer (see issue on concerns and considerations here), particularly regarding an inability to install nano, several instructors have advocated the use of Atom as a text editor for workshops using git and unix shell.

It would be fairly easy to add these installation instructions to the workshop-template for instructors to select. We would be very interested for instructors in upcoming workshops to test this and report back on how it works!

[rgaiacs]

Because many instructors advocated to keep nano I worked on #391 that will conflict with this issue.

[fmichonneau]

Now that nano is distributed by Git Bash, I think we can close this.