Add GitHub Actions workflow (#300)

.github/workflows/main.yml

@@ -0,0 +1,33 @@
+name: Maven CI
+
+on:
+  push:
+    branches: [ master ]
+  pull_request:
+    branches: [ master ]
+
+jobs:
+  build_and_test:
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Set up JDK 16
+      uses: actions/setup-java@v1
+      with:
+        java-version: 16
+
+    - name: Cache maven packages to speed up build
+      uses: actions/cache@v1
+      with:
+        path: ~/.m2
+        key: ${{ runner.os }}-m2-${{ hashFiles('**/pom.xml') }}
+        restore-keys: ${{ runner.os }}-m2
+
+    - name: Build with Maven
+      run: mvn -B package --file pom.xml
+
+    - uses: actions/upload-artifact@v1
+      with:
+        name: Extras
+        path: target/Extras.jar