Add a GitHub action to publish The Book (#877)

.github/workflows/book.yml

@@ -0,0 +1,30 @@
+name: mdbook
+
+on:
+  push:
+    branches:
+      - master
+
+jobs:
+  build-and-upload-to-s3:
+    runs-on: ubuntu-18.04
+    steps:
+      - uses: actions/checkout@master
+
+      - name: Setup mdBook
+        uses: peaceiris/actions-mdbook@v1
+        with:
+          mdbook-version: '0.3.5'
+
+      - run: mdbook build
+        working-directory: book
+
+      - uses: jakejarvis/s3-sync-action@be0c4ab89158cac4278689ebedd8407dd5f35a83
+        with:
+          args: --follow-symlinks --delete
+        env:
+          AWS_S3_BUCKET: ${{ secrets.AWS_S3_BOOK_BUCKET }}
+          AWS_ACCESS_KEY_ID: ${{ secrets.AWS_ACCESS_KEY_ID }}
+          AWS_SECRET_ACCESS_KEY: ${{ secrets.AWS_SECRET_ACCESS_KEY }}
+          AWS_REGION: 'ap-southeast-2'
+          SOURCE_DIR: 'book/book'