Add workflow for docs.

.github/workflows/deploy_docs.yml

@@ -0,0 +1,33 @@
+name: Deploy Docs
+
+on:
+  push:
+    branches:
+      - master
+    tags:
+      - v*
+    paths-ignore:
+      - 'CHANGELOG.md'
+jobs:
+  deploy:
+    runs-on: ubuntu-16.04
+    steps:
+      - uses: actions/checkout@v2
+      - uses: MeilCli/setup-crystal-action@v4.0.1
+        with:
+          crystal_version: 1.0.0
+          shards_version: 0.14.1
+      - name: Install dependencies
+        run: shards install
+      - name: Run crystal doc
+        run: crystal doc
+      - name: Extract branch name
+        shell: bash
+        run: echo "##[set-output name=branch;]$(echo ${GITHUB_REF#refs/heads/})"
+        id: extract_branch
+      - name: Deploy to gh-pages
+        uses: peaceiris/actions-gh-pages@v3.7.0-8
+        with:
+          personal_token: ${{ secrets.GITHUB_TOKEN }}
+          publish_dir: docs
+          destination_dir: ${{ steps.extract_branch.outputs.branch }}