Added features to document definition.

src/document.rs

@@ -13,3 +13,8 @@ pub use definition::{DocDef, DocFuncType};
 pub use field::{Field, FieldType, MissingTranslation};
 pub use record::{Record, Records};
 pub use session::Session;
+
+#[derive(Clone, Debug, Eq, Hash, PartialEq)]
+enum DocFeature {
+    System,
+}