Update tutorial and template.

recipes/002_registration.py

@@ -16,9 +16,9 @@
 |     └── YOUR_PACKAGE_NAME (you need to create this directory and its contents)
 |         ├── YOUR_ALGORITHM_NAME.py
 |         ├── __init__.py
-|         ├── (example.py)
 |         ├── README.md
 |         ├── LICENSE
+|         ├── (example.py)
 |         ├── (requirements.txt)
 |         └── (images)
 |             ├──  (thumbnail.png)
@@ -34,9 +34,9 @@
 
 - `YOUR_ALGORITHM_NAME.py`: The implementation of your algorithm.
 - `__init__.py`: An initialization file. This file must implement your algorithm or import its implementation from another file, e.g., `YOUR_ALGORITHM_NAME.py`.
-- `example.py`: This file should contain a simple example of how to use your algorithm (Example: `example.py for Simulated Annealing Sampler <https://github.com/optuna/optunahub-registry/blob/main/package/samplers/simulated_annealing/example.py>`_).
 - `README.md`: A description of your algorithm. This file is used to create an `web page of OptunaHub <https://hub.optuna.org/>`_. Let me explain the format of the `README.md` file later.
 - `LICENSE`: A license file. This file must contain the license of your algorithm. It should be the MIT license in the alpha version of OptunaHub.
+- `example.py`: This is optional. This file should contain a simple example of how to use your algorithm (Example: `example.py for Simulated Annealing Sampler <https://github.com/optuna/optunahub-registry/blob/main/package/samplers/simulated_annealing/example.py>`_).
 - `requirements.txt`: This is optional. A file that contains the additional dependencies of your algorithm. If there are no additional dependencies other than Optuna and OptunaHub, you do not need to create this file.
 - `images`: This is optional. A directory that contains images. The images in this directory will be used the `web page of OptunaHub <https://hub.optuna.org/>`_. `thumbnail.png` will be used as a thumbnail in the web page. Note that `README.md` can also refer to image files, e.g. `images/screenshot.png`,  in this directory.
 

template/README.md

@@ -37,3 +37,21 @@ Please fill in the code snippet to use the implemented feature here.
 ## Others
 Please fill in any other information if you have here by adding child sections (###).
 If there is no additional information, this section can be removed.
+
+<!--
+For example, you can add sections to introduce a corresponding paper.
+
+### Reference
+Takuya Akiba, Shotaro Sano, Toshihiko Yanase, Takeru Ohta, and Masanori Koyama. 2019.
+Optuna: A Next-generation Hyperparameter Optimization Framework. In KDD.
+
+### Bibtex
+```
+@inproceedings{optuna_2019,
+    title={Optuna: A Next-generation Hyperparameter Optimization Framework},
+    author={Akiba, Takuya and Sano, Shotaro and Yanase, Toshihiko and Ohta, Takeru and Koyama, Masanori},
+    booktitle={Proceedings of the 25th {ACM} {SIGKDD} International Conference on Knowledge Discovery and Data Mining},
+    year={2019}
+}
+```
+-->