DOC: note time taken for E. coli dotplot in README

this was already noted in the part of the tutorial that generates
this figure, but let's note it in the README also

README.md

@@ -65,6 +65,7 @@ from matplotlib import pyplot
 # (skipping the part where I loaded the genomes into memory as e1s and e2s...)
 
 # Create the matrix (leaving binary=True by default)
+# This takes about 3 minutes on a laptop with 8 GB of RAM
 em = wotplot.DotPlotMatrix(e1s, e2s, 20, verbose=True)
 
 # Visualize the matrix using matplotlib's spy() function