Define installation process

README.md

@@ -1,7 +1,12 @@
 # stitchify
 Convert pixel art to cross-stitch patterns
 
-Requires Python 3, PIL
+Requires Python 3.
+
+Before running stitchify, do the following (on Linux):
+
+```> sudo apt-get install gtk+-3.0
+> pip3 install argparse gooey pillow```
 
 This tool assumes that 1px = 1 stitch.