Add scheduled update for sub trees

.github/workflows/refresh-csl-subtrees.yml

@@ -0,0 +1,25 @@
+name: Refresh Citation Style Language Files
+
+on:
+  schedule:
+    # run on 1th and 15th of each month
+    - cron: '1 2 1,15 * *'
+
+jobs:
+  publish:
+    name: Refresh Citation Style Language Files
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout source
+        uses: actions/checkout@v1
+      - name: Add csl-styles remote
+        run: git remote add -f csl-styles https://github.com/citation-style-language/styles.git
+      - name: Update csl-styles
+        run: git subtree pull --prefix src/main/resources/csl-styles csl-styles master --squash
+      - name: Add csl-locales remote
+        run: git remote add -f csl-locales https://github.com/citation-style-language/locales.git
+      - name: Update csl-locales
+        run: git subtree pull --prefix src/main/resources/csl-locales csl-locales master --squash
+      - uses: ad-m/github-push-action@master
+        with:
+          github_token: ${{ secrets.GITHUB_TOKEN }}