demo of how to add .gitignore, what it does

[abartlett004]

would be nice to show students how to add a .gitignore file to their repo, perhaps using .DS_Store files as an example. would be able to show students how, pre-gitignore, github desktop is tracking .DS_Store files, and after creation of .gitignore and adding .DS* , .DS_Store files are not committed and do not end up in Github

could also do this with a word doc or powerpoint, to show students what changes to those files look like from git's point of view and then prevent those ugly changes from being committed