Add github build action

.github/workflows/gradle.yml

@@ -0,0 +1,21 @@
+name: Java CI
+
+on: [push]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v1
+      with: 
+        submodules: recursive
+    - name: Set up JDK 1.8
+      uses: actions/setup-java@v1
+      with:
+        java-version: 1.8
+    - name: Build with Gradle
+      run: ./gradlew build
+    - uses: actions/upload-artifact@v2-preview
+      with:
+        name: artifact
+        path: build/libs/*