Add GitHub Pages deployment workflow

.github/workflows/deploy.yml

@@ -0,0 +1,38 @@
+name: deploy
+on: [push]
+jobs:
+  build:
+    name: Deploy 'grain'
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: borales/actions-yarn@v3.0.0
+        with:
+          cmd: install # will run `yarn install` command
+      - uses: borales/actions-yarn@v3.0.0
+        with:
+          cmd: build # will run `yarn build` command
+
+      - name: Upload production-ready build files
+        uses: actions/upload-artifact@v2
+        with:
+          name: production-files
+          path: ./dist
+  deploy:
+    name: Deploy
+    needs: build
+    runs-on: ubuntu-latest
+    if: github.ref == 'refs/heads/main'
+
+    steps:
+      - name: Download artifact
+        uses: actions/download-artifact@v2
+        with:
+          name: production-files
+          path: ./dist
+
+      - name: Deploy to GitHub Pages
+        uses: peaceiris/actions-gh-pages@v3
+        with:
+          github_token: ${{ secrets.GITHUB_TOKEN }}
+          publish_dir: ./dist