Update README.md

README.md

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 # nfactor-decomp
-N-factor decomposition code implementing the Sun (1998) method
+N-factor decomposition code implementing the Sun (1998) method.
+
+## Usage:
+Use two lists of length `n`: one for the start year, the other for end year.
+
+```
+decomposition(n, values_start, values_end)
+```
+
+This returns the value at the start (product of `values_start`), the value at the end (product of `values_end`) and a list of `n` values, which are the differences due to each factor. The sum of the differences is equal to the difference between the value at the end and the value at the start.
+
+Make sure to import `numpy` and `itertools`:
+```
+import numpy as np
+import itertools
+```