Add README

README.md

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+# ʜƨiloꟼ
+### RPN calculator in haskell
+
+```sh
+$ hsilop
+1 2 sqrt / acos
+ꟼ∘ 0.7853981634
+
+5 8 9 2 * cos ^ - 2 0.452 tan / +
+ꟼ∘ 5.1717299516
+
+e pi ^ pi -
+ꟼ∘ 19.9990999792
+
+1 + 2
+Ꞥ∘ syntax error
+```
+
+## License
+
+Dual licensed under the MIT and GPL licenses:  
+http://www.opensource.org/licenses/mit-license.php  
+http://www.gnu.org/licenses/gpl.html