Add missing exercise README

Author: jussienko

numpy/reference-copy/README.md

@@ -0,0 +1,12 @@
+Investigate the behavior of the statements below by looking at the values of
+the arrays a and b after assignments:
+
+```
+a = np.arange(5)
+b = a
+b[2] = -1
+b = a[:]
+b[1] = -1
+b = a.copy()
+b[0] = -1
+```