sets up github build action.

.github/workflows/ci.yml

@@ -0,0 +1,23 @@
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+
+jobs:
+  build:
+    strategy:
+      matrix:
+        os: [macos-latest, ubuntu-latest, windows-latest]
+    runs-on: ${{ matrix.os }}
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+        with:
+          submodules: true
+      - name: Install Node.js
+        uses: actions/setup-node@v1
+        with:
+          node-version: 10.x
+      - run: yarn install
+      - run: yarn build