Add exercise assignment for Input and output

Author: mlouhivu

numpy/input-output/README.md

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+## Input and output
+
+File [xy-coordinates.dat](xy-coordinates.dat) contains a list of (x,y) value
+pairs. Read the data with `numpy.loadtxt()`. Add then 2.5 to all y values and
+write the new data into a file using `numpy.savetxt()`.