docs(sync-files): add installation instructions (#287)

Signed-off-by: M. Fatih Cırıt <[email protected]>

README.md

@@ -11,7 +11,6 @@ This workflow checks if a certain secret is set.
 #### Usage
 
 ```yaml
-jobs:
 jobs:
   check-secret:
     uses: autowarefoundation/autoware-github-actions/.github/workflows/check-secret.yaml@v1

sync-files/README.md

@@ -7,6 +7,35 @@ It uses [peter-evans/create-pull-request](https://github.com/peter-evans/create-
 
 Note that you need `workflow` permission for the token if you copy workflow files of GitHub Actions.
 
+## Initial setup (within `autowarefoundation` org)
+
+This action uses the <https://github.com/apps/awf-autoware-bot> app to create pull requests.
+
+### Secrets
+
+For this action to use this bot, it requires the following secrets:
+
+- `APP_ID`: The app ID of the bot.
+  - Can be found in [awf-autoware-bot App Settings](https://github.com/organizations/autowarefoundation/settings/apps/awf-autoware-bot) General, About section.
+- `PRIVATE_KEY`: The private key of the bot.
+  - Can be found/generated in [awf-autoware-bot App Settings](https://github.com/organizations/autowarefoundation/settings/apps/awf-autoware-bot) General, Private keys section.
+
+Then these secrets need to be set in the repository settings.
+
+- Target Repository GitHub Page, Settings, Secrets and variables, Actions, New repository secret, `APP_ID` and `PRIVATE_KEY`
+
+### App settings
+
+Also, you need to install the app to the target repository.
+
+- [awf-autoware-bot App Settings](https://github.com/organizations/autowarefoundation/settings/apps/awf-autoware-bot) Install App section, Repository access, Only select repositories, add the target repository.
+
+### Additional repository settings
+
+For this action to work properly, the following settings are required in the target repository.
+
+- Target Repository GitHub Page, Settings, General, Pull Requests, Allow auto-merge # Enable
+
 ## Usage
 
 ```yaml