Add GitHub CI.

.github/workflows/ci.yml

@@ -0,0 +1,27 @@
+name: CI
+
+on:
+  push:
+    branches: [ master ]
+  pull_request:
+    branches: [ master ]
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+      - name: Setup Java
+        uses: actions/setup-java@v4
+        with:
+          distribution: 'temurin'
+          java-version: '17'
+      - name: Setup Gradle
+        uses: gradle/actions/setup-gradle@v3
+      - name: Run clean build javadoc
+        run: ./gradlew clean build javadoc

README.md

@@ -1,4 +1,7 @@
 # ![C Thing Software](https://www.cthing.com/branding/CThingSoftware-57x60.png "C Thing Software") checkstyle-checks
+
+[![CI](https://github.com/cthing/checkstyle-checks/actions/workflows/ci.yml/badge.svg)](https://github.com/cthing/checkstyle-checks/actions/workflows/ci.yml)
+
 Library of custom checkers for Checkstyle.
 
 ### Usage