In addition to architecture, I'd strongly recommend to add "GLOSSARY". In many software projects, a certain common noun has a project specific meaning, and it's confusing to a first-time reader who has to figure out which one is a special term. In my own PDF parser project, for example, "stream", "trailer", and "literal" are PDF-specific terms that are different from its normal usage. A glossary also helps introducing basic concepts used in the ARCHITECTURE.