Update Contributing file with doxygen

CONTRIBUTING.md

@@ -5,3 +5,7 @@ We use the Google C++ Style Guide that can be found at https://google.github.io/
 # Testing
 
 We use the test framework [catch2](https://github.com/catchorg/Catch2) for writing and running the test cases.
+
+# Documentation
+
+We use [Doxygen](http://www.doxygen.org/) for writing and generating the documentation.