Add example code

Author: sharkdp

README.md

@@ -4,6 +4,23 @@ A library for calculations with arbitrary length integers.
 This is a simple wrapper around [BigInteger.js](https://github.com/peterolson/BigInteger.js)
 by [Peter Olson](https://github.com/peterolson).
 
+## Example
+
+```purescript
+x = fromInt 42
+y = (fromJust <<< fromBase 16) "fe45aab12"
+mersenne10 = (fromInt 2) `pow` (fromInt 89) - one
+
+> x `pow` x
+fromString "150130937545296572356771972164254457814047970568738777235893533016064"
+
+> toNumber (y * mersenne10)
+4.2248205181941055e+37
+
+> prime mersenne10
+true
+```
+
 ## Module documentation
 
 - [Data.BigInt](docs/Data/BigInt.md)