docs(readme): Add how to use this tool

New users can understand easily.

.github/README.md

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+<h1 align="center">OPML Generator</h1>
+
+<div align="center">
+
+**📰 Generate OPML file via TOML file easily**
+
+[![GitHub Release](https://img.shields.io/github/v/release/5ouma/opml-generator?style=flat-square)](https://github.com/5ouma/opml-generator/releases)
+![GitHub code size in bytes](https://img.shields.io/github/languages/code-size/5ouma/opml-generator?style=flat-square)
+![GitHub repo size](https://img.shields.io/github/repo-size/5ouma/opml-generator?style=flat-square)
+[![GitHub last commit](https://img.shields.io/github/last-commit/5ouma/opml-generator?style=flat-square)](https://github.com/5ouma/opml-generator/commit/HEAD)
+[![GitHub commit activity](https://img.shields.io/github/commit-activity/m/5ouma/opml-generator?style=flat-square)](https://github.com/5ouma/opml-generator/commits/main)
+<br />
+[![Test](https://img.shields.io/github/actions/workflow/status/5ouma/opml-generator/test.yml?label=test&style=flat-square)](https://github.com/5ouma/opml-generator/actions/workflows/test.yml)
+[![Gist Update](https://img.shields.io/github/actions/workflow/status/5ouma/opml-generator/gist-update.yml?label=Gist%20Update&style=flat-square)](https://github.com/5ouma/opml-generator/actions/workflows/gist-update.yml)
+[![pre-commit.ci status](https://results.pre-commit.ci/badge/github/5ouma/opml-generator/main.svg?style=flat-square)](https://results.pre-commit.ci/latest/github/5ouma/opml-generator/main)
+
+</div>
+
+<br /><br />
+
+## 📊 Usage
+
+OPML Generator has 2 ways to convert TOML to OPML.
+
+1. ### 💻 On Local
+
+   1. 📰 Prepare your TOML file (Reference: [feeds.toml](./assets/example/feeds.toml)).
+
+   2. 🦕 Run Deno Task
+
+      ```shell
+      deno task gen
+      ```
+
+   3. 🗞️ Outputs are stored in the `outputs` directory separated by `lists`.
+
+2. ### 🐙 On GitHub Actions
+
+   1. 🗒️ Create [a Gist](https://gist.github.com):
+      - includes [`feeds.toml`](./assets/example/feeds.toml).
+      - for outputs.
+   2. 🔑 Create [a Personal Access Token] with the `Gist` permission.
+   3. 🍴 [Fork this repository](https://github.com/5ouma/opml-generator/fork)
+   4. 🔏 Add Actions secrets
+      from `Settings > Secrets and variables | Actions` in the sidebar:
+      - `TOML_GIST_ID`: Gist ID for [`feeds.toml`](./assets/example/feeds.toml)
+      - `OPML_GIST_ID`: Gist ID for outputs
+      - `TOKEN`: Personal Access Token
+   5. ✅ Enable GitHub Actions by checking `Allow all actions and reusable workflows`
+      from `Settings > Actions | General` in the sidebar.
+
+   🎉 Automatically update every 0 a.m. UTC
+
+[a Personal Access Token]: https://github.com/settings/tokens
+
+<br /><br />
+
+## 🗞️ Subscribe OPML
+
+For Inoreader, please see the official blog, [**OPML subscriptions**](https://www.inoreader.com/blog/2014/05/opml-subscriptions.html).
+<br />
+For other RSS readers, please see each help page.
+
+<br /><br />
+
+## 🆘 Help
+
+- [**⚠️ Issues**]: Feature Requests or Bug Reports
+- [**💬 Discussions**]: General Chats or Questions
+- [**🛡️ Security Advisories**]: Security Issues that should not be public
+
+[**⚠️ Issues**]: https://github.com/5ouma/opml-generator/issues/new/choose
+[**💬 Discussions**]: https://github.com/5ouma/opml-generator/discussions/new/choose
+[**🛡️ Security Advisories**]: https://github.com/5ouma/opml-generator/security/advisories/new
+
+<br /><br />
+
+## 🎽 Contributing
+
+I happily welcome your contributions!
+Before you contribute,
+I would recommend reading [CONTRIBUTING.md](./CONTRIBUTING.md)
+for a better development experience.

.github/assets/example/feeds.toml

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+[[lists]]
+name = "list 1"
+
+[[lists.feeds]]
+title = "feed 1"
+xmlUrl = "https://feed1.com/rss"
+[[lists.feeds]]
+title = "feed 2"
+xmlUrl = "https://feed2.com/rss"
+
+
+[[lists]]
+name = "list 2"
+
+[[lists.feeds]]
+title = "feed 1"
+xmlUrl = "https://feed1.com/rss"
+[[lists.feeds]]
+title = "feed 2"
+xmlUrl = "https://feed2.com/rss"