creates initial README

Author: attwoodn

README.md

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+<div align="center">
+    # CPP Expression Tree
+    
+    Header-only C++14 library for creating and evaluating logical expression trees. 
+    
+    Inspired by m-peko/booleval.
+</div>
+
+<br/>
+
+This project is under development and is subject to change. Project contributions and issue reports are welcome. The more the merrier! 
+( ... well, hopefully not too many bug reports)
+
+## Table of Contents
+
+* [A Quick Example](#a-quick-example)
+* [Types of Expression Tree Nodes](#types-of-expression-tree-nodes)
+    * [Expression Tree Leaf Nodes](#expression-tree-leaf-nodes)
+    * [Expression Tree Op Nodes](#expression-tree-op-nodes)
+* [Logical Operators](#logical-operators)
+* [Boolean Operators](#boolean-operators)
+* [Using this Library](#using-this-library)
+* [Compilation](#compilation)
+    * [Running the Unit Tests](#running-the-unit-tests)
+
+## A Quick Example
+
+```cpp
+// some code here
+// imagine we have some class struct whatever ...
+// we can create an expression_tree for evaluating the contents of instances of this struct class whatever
+```
+
+Below is a tree diagram showing the content of the expression_tree that was created in the example code above:
+
+<p align="center">
+    <img src="docs/booleval-tree.png"/>
+</p>
+
+## Types of Expression Tree Nodes
+
+There are two types of expression tree nodes: leaf nodes and op nodes. 
+
+### Expression Tree Leaf Nodes
+
+Expression tree leaf nodes contain individual, actionable expressions against which a class/struct instance is evaluated. Expression tree leaf nodes are only ever found at the extremities of the expression tree.
+
+### Expression Tree Op Nodes
+
+Expression tree op nodes contain a boolean operation (AND/OR) and have references to a left child node and a right child node. The child nodes may be expression tree leaf nodes, expression tree op nodes, or a permutation of the two. Expression tree op nodes are only ever found in the inner part of the tree. An expression tree op node is always a parent node and it always has two child nodes.
+
+## Logical Operators
+
+There are several logical operator functions defined in the namespace `attwoodn::expression_tree::op`, which can be used to create individual expressions within the tree. The included operators are:
+ * equals
+ * not_equals
+ * less_than
+ * greater_than
+
+Users of the library can also easily define their own logical operators and use them when creating expressions. Here is an example of how a user might create their own operator functions and use them in an expression:
+
+```cpp
+// some code here
+// is_pointer functions: always returns false (value parameters), always returns true (pointer parameters)
+```
+
+## Boolean Operators
+
+Boolean operators are used to chain individual expression tree nodes together. There are two boolean operators that can be used: `AND` and `OR`. These boolean operators are accessible via function calls on the expression nodes. Calling these functions generates a new expression tree node which becomes the parent of the nodes on either side of the boolean operator
+
+```cpp
+// some code here
+// shows how nodes are chained with AND and OR functions
+```
+
+A complex expression tree can be created by calling these functions to chain multiple expression tree nodes together.
+
+## Using this Library
+
+To include this library in your project, simply copy the content of the `include` directory into the `include` directory of your project. That's it! Now where did I put that Staples "Easy" button...?
+
+## Compiling
+
+First, clone the git repository and navigate into the local copy. Once you're there, run the following commands:
+
+```
+mkdir build && cd build
+cmake ..
+make
+```
+
+### Running the Unit Tests
+
+After cloning and compiling the project, navigate to the build directory that was created. Enable the `BUILD_TESTING` CMake flag if it is not already enabled. Finally, run:
+
+`ctest .`
+
+This will execute the unit tests and provide a pass/fail indication for each one.