updates README.md

Author: attwoodn

README.md

@@ -37,10 +37,12 @@ Below is a tree diagram showing the content of the expression_tree that was crea
 
 ## Creating Expression Trees
 
-The `expression_tree` class is a templated, RAII container class that takes ownership of user-defined expressions. The template parameter of `expression_tree` is the type of object that the `expression_tree` can evaluate. Assuming there is a user-defined class named `my_type` to be evaluated, the templated `expression_tree` type would look like this: `expression_tree<my_type>`. The template parameter of `expression_tree` cannot be a primitive type, like `int`, `char`, or `double`.
+The `expression_tree` class is a templated, RAII container class that takes ownership of user-defined expressions. The template parameter of `expression_tree` is the type of object that the `expression_tree` can evaluate. Assuming there is a user-defined class named `my_type`, the templated `expression_tree` type would look like this: `expression_tree<my_type>`. The template argument of `expression_tree` cannot be a primitive type, like `int`, `char`, or `double`.
 
 An `expression_tree` cannot be default constructed - it must be initialized with an expression. Users can easily and intuitively define expressions using one of the `make_expr` helper functions found in the namespace `attwoodn::expression_tree`. `make_expr` generates heap-allocated pointers to expression tree nodes and returns them. As such, the returned expression tree node pointers should be managed carefully. If the returned pointers are not wrapped in an `expression_tree` or a smart pointer, they will need to be explicitly `delete`d by the calling code. 
 
+// TODO document the thre make_expr functions
+
 Here are some examples of how you might handle the return value from one of the `make_expr` helper functions:
 ```cpp
 #include <attwoodn/expression_tree.hpp>