add GitHub Actions CI

.github/workflows/ci.yml

@@ -0,0 +1,24 @@
+name: CI
+
+on:
+  pull_request:
+    branches:
+      - master
+  push:
+    branches:
+      - master
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        java: [ '8', '11', '16' ]
+    steps:
+    - uses: actions/checkout@v2
+    - name: Set up JDK
+      uses: actions/setup-java@v1
+      with:
+        java-version: ${{ matrix.java }}
+    - name: Build
+      run: mvn --no-transfer-progress -B -P ci verify