Use github actions instead of travis CI (#180)

.github/workflows/check.yml

@@ -0,0 +1,15 @@
+name: check
+on: [push]
+jobs:
+  markdownlint:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: Use Node.js 12
+        uses: actions/setup-node@v1
+        with:
+            node-version: 12
+      - name: Check *.md files by `markdownlint`
+        run: |
+          npm install -g markdownlint-cli
+          markdownlint *.md