update readme

README.md

@@ -22,7 +22,8 @@
 
 ### Run Source Injection
 * Inject souces onto the L1 calibrated images
-* modify the corresponding ```config_injection.yaml``` file to configure all running options
+* Modify the corresponding ```config_injection.yaml``` file to configure all running options
+* An object catalog class which inherits from ```observation_sim.mock_objects.CatalogBase``` needs to be implemented and imported in ```injection.injection_pipeline.py```. This is to construct a list of to be injected ```observation_sim.mock_objects.MockObject```.
 ```
     python ./injection_pipeline/injection_pipeline.py /path/to/config_injection.yaml
 ```