added doc for how to add a new command group (#575)

Co-authored-by: Joe Zhou <jiazho@microsoft.com>

doc/03-manual-customizations.md

@@ -27,6 +27,32 @@
 * add **custom.py** file in **manual** folder if it's not exist and provide the implementation for your new command
 ![sample image](images/manual-override-add-command3.png)
 
+## How to add a new command group
+* Add **...\manual\command.py** file if it's not exist and provide your new command group and its commands as below. The client_factory is optional and is needed when you want to use the special arguments such as `cmd`. You can take **...\generated\command.py** as a reference.
+```python
+def load_command_table(self, _):
+
+    from azext_ml.generated._client_factory import cf_ml_cl
+    with self.command_group('portal newgroup', client_factory=cf_ml_cl) as g:
+        g.custom_command('create', 'newgroup_create')
+        g.custom_command('update', 'newgroup_show')
+        g.custom_command('delete', 'newgroup_delete')
+``` 
+* Add **...\manual\custom.py** file if it's not exist and implement your new command. You can take **...\generated\custom.py** as a reference.
+```python
+def newgroup_create(cmd, arg0, arg1, ...):
+    ...
+def newgroup_show(cmd, arg0, arg1, ...):
+    ...
+def newgroup_delete(cmd, arg0, arg1, ...):
+    ...
+```
+* As above, you can add or edit **...\manual\\_params.py** file to implement arguments for your new commands. 
+* Make sure you can run these commands in the terminal of your choice:
+```bash
+az portal newgroup create
+```
+
 ## Overriding Actions
 
 It's also possible to override actions or add any new actions by adding **actions.py** file in **manual** folder.