Note about embedded documents added to mongodb extension readme

extensions/mongodb/README.md

@@ -184,6 +184,37 @@ $models = $provider->getModels();
 ```
 
 
+Working with embedded documents
+-------------------------------
+
+This extension does not provide any special way to work with embedded documents (sub-documents).
+General recommendation is avoiding it if possible.
+For example: instead of:
+
+```
+{
+	content: "some content",
+	author: {
+		name: author1,
+		email: author1@domain.com
+	}
+}
+```
+
+use following:
+
+```
+{
+	content: "some content",
+	author_name: author1,
+	author_email: author1@domain.com
+}
+```
+
+Yii Model designed assuming single attribute is a scalar. Validation and attribute processing based on this suggestion.
+Still any attribute can be an array of any depth and complexity, however you should handle its validation on your own.
+
+
 Using GridFS
 ------------