feat: add README

README.md

@@ -0,0 +1,55 @@
+# GitHub Action for TestLens
+
+The `setup-testlens` action connects a Maven or Gradle build to [TestLens](https://testlens.app).
+
+## Prerequisites
+
+The [TestLens GitHub App](https://github.com/apps/testlens-app) needs to be installed on the repository.
+
+> [!IMPORTANT]
+> **TestLens is currently in private beta.**
+> Therefore, an extra step is required to onboard a new GitHub organization.
+> If you’re interested in trying out TestLens, please [contact us via the website](https://testlens.app/contact/).
+
+## Setup for Gradle
+
+For Gradle, the action should be added right after the `setup-gradle` action in all workflow files that should be instrumented:
+
+```yml
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v6
+      - uses: actions/setup-java@v5
+        with:
+          distribution: temurin
+          java-version: 8 # or later
+      - uses: gradle/actions/setup-gradle@v5
+      - uses: testlens-app/setup-testlens@v5
+      - run: ./gradlew build
+```
+
+The action writes a Gradle init script that instruments all `Test` tasks.
+
+## Setup for Maven
+
+For Maven, the action needs to be added before the first call to `mvn` that should be instrumented.
+We recommend adding it right after `actions/setup-java` or a similar action that ensures the required version of Java is available.
+
+```yml
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v6
+      - uses: actions/setup-java@v5
+        with:
+          distribution: temurin
+          java-version: 8 # or later
+      - uses: testlens-app/setup-testlens@v5
+      - run: mvn verify
+```
+
+The action expects the root parent POM to be present in the root directory of the repository.
+It alters the POM by adding a profile that instruments all executions of the `maven-surefire-plugin`.