Updated readme and example on calculator_tree

README.md

 To run tests: `python test_calculator_tree.py`
 Main code resides in `tree_traversing.py`
 
-Main function that can calculate the strings: `calculate(string)`
\ No newline at end of file
+Main function that can calculate the strings: `calculate(string)`
+
+e.g
+
+`calculate(-4-7*3).calculate_below()` will result in `-25`
\ No newline at end of file

calculator_tree.py

@@ -109,4 +109,4 @@ def calculate(expr, binary_tree=None):
 if __name__ == "__main__":
     expression = "-4-7*3"
 
-    print(calculate(expression))
+    print(calculate(expression).calculate_below())