Documenting Open Babel is an important, and never-ending task. This includes clear documentation on the interfaces of particular classes and methods, (i.e., the API documentation) but also tutorials and examples of using the Open Babel library to accomplish clear tasks.
Beyond outside documentation described above, as an open-source project involving many, many contributors, the internal code should be clearly commented and easy to read (in English, preferably, since this is the common language of developers on the project).
Adding New Code
WRITE THE DOCUMENTATION, THEN CODE TO THE SPECS.
You should never, ever start writing code unless you've specified, clearly and exactly, what your code will do. This makes life easier for you (i.e., you know exactly what the code should do), and for others reading your code.
This mantra also facilitates writing [].