Add instruction to use requirements.txt

[HideakiImamura]

Please read the following and if you agree, please click the checkbox below.

1. By making a contribution to this project, I certify that:
   a. The contribution was created in whole or in part by me and I have the right to submit it under the MIT license indicated in the file; or
   b. The contribution is based upon previous work that, to the best of my knowledge, is covered under an appropriate open-source license and I have the right under that license to submit that work with modifications, whether created in whole or in part by me, under the MIT license, as indicated in the file; or
   c. The contribution was provided directly to me by some other person who certified (a), (b) or (c) and I have not modified it.

2. By making a contribution to this project, I agree that the MIT license applies to the contribution and the contribution is used by other people under the MIT license.

• I agree to the above conditions.

Please read the tutorial guide to register your implemented algorithm in OptunaHub.

Motivation

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Description of the changes

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[y0z]

LGTM