Add GH workflow to check code style & license

Author: swallez

.github/workflows/checkstyle.yml

@@ -0,0 +1,18 @@
+name: Code style and license headers
+
+on: [pull_request]
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: Set up JDK 11
+        uses: actions/setup-java@v2
+        with:
+          java-version: '11'
+          distribution: 'adopt'
+
+      - name: Check style and license headers
+        run: |
+          ./gradlew checkstyleMain checkstyleTest

CONTRIBUTING.md

@@ -13,7 +13,7 @@ before we can accept pull requests from you.
 
 ### Prerequisites
 
-You need at least Java 8. The project is built with `Gradle`. You don't have to install it, and can use the provided `gradlew` that will install the required version if needed.
+You need at least Java 11 to build the project, even though the code targets Java 8. The project is built with `Gradle`. You don't have to install it, and can use the provided `gradlew` that will install the required version if needed.
 
 Docker is used for some tests to spawn an Elasticsearch server.