Fix readme info gfx

I noticed that the text was talking about a remote named 'swc', but the images refer to a remote named 'swcarpentry'. Since I noticed some other minor issues with the images, I decided to change the images and not the text. It should be easier to recreate the three images from the SVG file now. The bottom layer contains everything in step 1, the middle layer everything additional for step 2 and all the rest for step 3 is in the top layer. In addition I re-grouped stuff. See single commits for more details.