Add javadoc auto-build on master

.github/workflows/javadoc.yml

@@ -0,0 +1,27 @@
+name: Build and deploy Javadoc
+
+on:
+  push:
+    branches: [master]
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Set up JDK 11
+      uses: actions/setup-java@v1
+      with:
+        java-version: 1.11
+    - name: Build javadoc
+      run: gradle javadoc
+      
+    - name: Deploy javadoc to its assigned branch
+      uses: s0/git-publish-subdir-action@develop
+      env:
+        REPO: self
+        BRANCH: javadoc
+        FOLDER: docs
+        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}