caching java dependencies in ci workflow

Author: csviri

.github/workflows/ci.yml

@@ -13,6 +13,12 @@ jobs:
         java: [8, 11]
     steps:
       - uses: actions/checkout@v2
+      - uses: actions/cache@v1
+        with:
+          path: ~/.m2/repository
+          key: ${{ runner.os }}-maven-${{ hashFiles('**/pom.xml') }}
+          restore-keys: |
+            ${{ runner.os }}-maven-  
       - name: Set up JDK
         uses: actions/setup-java@v1
         with: