Add publish action.

.github/workflows/pages.yml

@@ -0,0 +1,30 @@
+name: Update GitHub Pages
+
+on:
+  push:
+    branches:
+      - master
+
+jobs:
+  deploy:
+    runs-on: ubuntu-latest
+
+    steps:
+    - name: Checkout repository
+      uses: actions/checkout@v2
+
+    - name: Setup Rust environment
+      uses: actions/setup-rust@v1
+      with:
+        toolchain: stable
+
+    - name: Build book
+      run: |
+        cargo install mdbook
+        mdbook build
+
+    - name: Deploy to GitHub Pages
+      uses: peaceiris/actions-gh-pages@v3
+      with:
+        github_token: ${{ secrets.GITHUB_TOKEN }}
+        publish_dir: ./book