Add instructions for viewing hidden files on Mac OS

[aaarcher-usgs]

Issue #16: Using a gitignore file, the .gitignore file is hidden on Macs, so step 1 is challenging if you're not already familiar with viewing the hidden files. I suggest possible modifications (in bold), worded and formatted as you see fit:

Action: Add a file and then gitignore it.

1. First, open the .gitignore file that is in your local directory using a text editor (for example, Notepad++). It should be completely empty. If you are using a Mac, you may need to click Command + Shift + . (period) while in the local directory in Finder to see and open the hidden .gitignore file.
2. Next, download this image (right click and choose "Save image as") and save as wss-wu-dryville.jpg in your ds-gitflows-[username] folder.
3. Now in Git Bash, run ls to "list" the items in your current working directory. You should see the following: dryville_story.md, README.md and wss-wu-dryville.jpg. Note that the .gitignore file doesn't show up with ls because it is technically a "hidden" file (starts with a .). To see the hidden file type ls -a.