Add explanation about figures

package/visualization/plot_hypervolume_history_with_rp/README.md

@@ -63,7 +63,7 @@ if __name__ == "__main__":
     fig.show()
 ```
 
-![Example](img/example.png "Example")
+![Example](images/example.png "Example")
 
 
 # Author Information

recipes/001_first.py

@@ -122,6 +122,9 @@ def objective(trial: optuna.trial.Trial) -> float:
 # |         ├── README.md
 # |         ├── __init__.py
 # |         ├── LICENSE
+# |         ├── images
+# |         │   ├──  thumbnail.png
+# |         │   └──  screenshot.png
 # |         └── YOUR_ALGORITHM_NAME.py
 #
 # If you implement not visualizarion feature but sampler, you should put your implementation in the `samplers` directory.
@@ -131,6 +134,7 @@ def objective(trial: optuna.trial.Trial) -> float:
 # - `README.md`: A description of your algorithm. This file is used to create an `web page of OptunaHub <TBD>`_. Let me explain the format of the `README.md` file later.
 # - `__init__.py`: An initialization file. This file must import your impelemented sampler from `YOUR_ALGORITHM_NAME.py`.
 # - `LICENSE`: A license file. This file must contain the license of your algorithm. It should be the MIT license in the alpha version.
+# - `images`: This is optional. A directory that contains images. You can put images that are used in the `web page of OptunaHub <TBD>`_. Note that the image file named `thumbnail.png` is used as a thumbnail in the web page. If you want to put other images like `screenshot.png` and use them in the `README.md` file, please use the relative path like `images/screenshot.png`.
 # - `YOUR_ALGORITHM_NAME.py`: Your implemented sampler.
 #
 # The `README.md` file must contain the following sections: