added contribution documentation

CONTRIBUTE.md

@@ -0,0 +1,9 @@
+### **How to contribute?** 
+
+* Open a pull request explaining your addition/fix.If it is an addition of a class or an algorithm please explain why it's useful and provide a url to a wikipedia page for the algorithm/data structure you're suggesting.
+
+* Implementation is not everyting, you have to provide test cases in the **tests** folder for **EVERY** function you implement.
+
+* Last but not least, you have to write some documentation in the **tutorial** folder for the users to know how to use your code. 
+
+We are happy to see you contribute :).