Minor Improvement to README

[smiths]

I went through the README again. I'm able to do an ascending and descending segmentation now! The video was definitely helpful. In the process of going through it, I noticed some minor things that we should address:

• we should add advice to the user to change the qualified slice factor if the image quality is poor
• if 2.0 is a good value for the qualified slice factor, we should start the value at 2.0, instead of 1.0.
• we should also have advice about the number of slices to skip, and we should have a reasonable default value
• the readme doesn't mention the sagittal phase. A bullet point about that should be added.
• we should include a warning that the sagittal phase will take a long time (maybe it was just me running it, but it look forever on my computer. I thought the program was in an infinite loop.)