Add diagram to readme

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 # pgm-interlace
 Vertically interlace *n* PGM frames, each one *height/n* pixels in width into
 one square image with a width the same as the height.
+
+By example, it would take two 2x4 images:
+
+	Image A:
+	  0 1
+	1 A B
+	2 C D
+	3 E F
+	4 G H
+
+	Image B:
+	  0 1
+	1 I J
+	2 K L
+	3 M N
+	4 O P
+
+combining them into one single 4x4 image from the command `pgm-interlace imageA imageB`
+
+	  0 1 2 3
+	0 A I B J
+	1 C K D L
+	2 E M F N
+	3 G O H P
+
+The order in which the frames are specified on the command line is critical for
+the reconstruction of the image. pgm-interlace has no way of telling in which
+order a handful of pgm frames should be interlaced unless you tell it how to
+stitch them together.