docs: update workflow instructions to yml

Author: wei

README.md

@@ -14,17 +14,22 @@ A GitHub Action for syncing between two independent repositories using **force p
 
 ### Github Actions
 ```
-action "repo-sync" {
-  uses = "wei/git-sync@master"
-  args = "$SOURCE_REPO $SOURCE_BRANCH $DESTINATION_REPO $DESTINATION_BRANCH"
-  env = {
-    SOURCE_REPO = ""
-    SOURCE_BRANCH = ""
-    DESTINATION_REPO = ""
-    DESTINATION_BRANCH = ""
-  }
-  secrets = ["SSH_PRIVATE_KEY"]
-}
+# File: .github/workflows/repo-sync.yml
+
+on: push
+jobs:
+  repo-sync:
+    runs-on: ubuntu-latest
+    steps:
+    - name: repo-sync
+      uses: wei/git-sync@master
+      env:
+        SOURCE_REPO: ""
+        SOURCE_BRANCH: ""
+        DESTINATION_REPO: ""
+        DESTINATION_BRANCH: ""
+        SSH_PRIVATE_KEY: ${{ secrets.SSH_PRIVATE_KEY }}
+      args: $SOURCE_REPO $SOURCE_BRANCH $DESTINATION_REPO $DESTINATION_BRANCH
 ```
 `SSH_PRIVATE_KEY` can be omitted if using authenticated HTTPS repo clone urls like `https://username:access_token@github.com/username/repository.git`.