Started writing model creation guidelines.

doc/model_guidelines.rst

+Adding new model backend
+========================
+
+If you want to implement new database backend to use with model views, follow steps from this guideline.
+
+    1. Create new class and derive it from flask.ext.adminex.model.BaseModel::
+
+        class DbModel(BaseModel):
+            pass
+
+    2. Implement following scaffolding methods::
+
+        - `scaffold_list_columns`
+            - Make sure you support `excluded_list_columns`
+        - `scaffold_sortable_columns`
+        - `scaffold_form`
+            - Make sure you support `excluded_form_columns`
+        - `init_search`
+
+        If your database does not support free-form search,
+        return `False` from the `init_search`.
+
+        If your database does not support sorting, override
+        `get_sortable_columns` and return empty dictionary.
+
+    3. Implement data access methods::
+
+        - `get_list`
+        - `get_one`
+
+    4. Implement model management methods
+        - `create_model`
+        - `update_model`
+        - `delete_model`