Add README.md for the Gradle plugin

Fixes #5804

Co-Authored-By: Guillaume Smet <guillaume.smet@gmail.com>

devtools/gradle/README.md

@@ -0,0 +1,46 @@
+Quarkus Gradle Plugin
+=====================
+
+Builds a Quarkus application, and provides helpers to launch dev-mode, the Quarkus CLI and the build of native images.
+
+Releases are published at https://plugins.gradle.org/plugin/io.quarkus .
+
+Functional Tests
+----------------
+
+To run the functional tests, run the following command:
+
+```bash
+./gradlew functionalTests
+```
+
+Local development
+-----------------
+
+1. Build the entire Quarkus codebase by running `mvn clean install -DskipTests -DskipITs` in the project root 
+    - This should install the Gradle plugin in your local maven repository.
+
+2. Create a sample project using the Maven plugin:
+
+```bash
+    mvn io.quarkus:quarkus-maven-plugin:999-SNAPSHOT:create \
+        -DprojectGroupId=org.acme \
+        -DprojectArtifactId=my-gradle-project \
+        -DclassName="org.acme.quickstart.GreetingResource" \
+        -DplatformArtifactId=quarkus-bom \
+        -Dpath="/hello" \
+        -DbuildTool=gradle
+```
+
+Follow the instructions in the [Gradle Tooling Guide](https://quarkus.io/guides/gradle-tooling) for more information about the available commands.
+
+Importing using Intellij
+-------------------------
+
+Disable "Maven Auto Import" for the Quarkus projects. Since the Gradle plugin has a pom.xml,
+IntelliJ will configure this project as a Maven project. You need to configure it to be a Gradle
+project. To do so, follow these instructions:
+
+
+1. Go to File -> Project Structure
+2. In Modules, remove the `quarkus-gradle-plugin` and re-import as a Gradle project.