update readme

README.md

@@ -3,6 +3,7 @@
     <img src="static/cartoon.webp" alt="Image" width="512" height="512">
 </p>
 
+Backprop is a library for automatically differentiation mathematical expressions.
 
 ## Usage
 Add backprop.cpp as include in your project. Start by including the header file in your cmake file.
@@ -15,7 +16,6 @@ Then include the header file in your source file.
 #include "backprop.h"
 ```
 
-
 ## Tests
 ```bash
 mkdir build