Simplify installation instructions (OSX)

[ozskywalker]

Have noticed that setup.py install is automatically pulling in Pillow from pip:

Processing dependencies for wallpaper-reddit==3.1.1
Searching for Pillow==4.3.0
Best match: Pillow 4.3.0
Adding Pillow 4.3.0 to easy-install.pth file

Using /usr/local/lib/python3.6/site-packages
Searching for olefile==0.44
Best match: olefile 0.44
Adding olefile 0.44 to easy-install.pth file

Using /usr/local/lib/python3.6/site-packages
Finished processing dependencies for wallpaper-reddit==3.1.1

Could you simply the install instructions by placing xcode-select --install into setup.py, and just have the user run setup.py install instead?

[markubiak]

So Mac OS X support went from "it should work" to some guy telling me "it works, here's how I did it" and adding it to the README. If you can make the edits to setup.py and make a PR I'm happy to submit

[ozskywalker]

No worries, I'll whip up a PR then... :)