Add Processing Gradle plugin README

Introduce java/gradle/README.md documenting the Processing Gradle plugin. Explains how to turn a Processing sketch into a Gradle project using the org.processing.java plugin (build.gradle.kts examples), available tasks (gradle sketch, export, present), dependency and sketchbook configuration, and how to compose multi-sketch projects. Includes examples for using other Gradle plugins and switching Processing versions.

Author: Stefterv

java/gradle/README.md

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+# Processing Gradle Plugin
+
+This folder contains the source for the Processing Gradle plugin.
+The plugin will transform any Processing sketch into a Gradle project for easy compilation and advanced features.
+
+## Usage
+
+Add the following files to any Processing sketch alongside the `.pde` files
+
+```kotlin
+// build.gradle.kts
+plugins {
+    id("org.processing.java") version "4.5.3" // version of Processing you would like to use.
+}
+
+// settings.gradle.kts - create the file but leave blank
+```
+
+This will make the Processing sketch a fully fledges Gradle project, usable with any editor that supports gradle.
+Including the `gradle` command if installed.
+
+The plugin will add the `sketch` command to the Gradle tasks lists, so run the sketch with `gradle sketch`, this will
+build and launch your sketch.
+
+The sketch can also be bundled into a standalone app by using the `gradle export` command.
+Or run in fullscreen with `gradle present`
+
+To include libraries into your sketch add `processing.sketchbook=/path/to/sketchbook` to a `gradle.properties` file in
+the same folder.
+
+To use any kind of dependency add as a normal gradle dependency, the plugin has already automatically added the Maven
+Central repository.
+
+```kotlin
+// build.gradle.kts
+plugins {
+    id("org.processing.java") version "4.5.3"
+}
+
+dependencies {
+    implementation("com.lowagie:itext:2.1.7")
+}
+```
+
+To use an older version of Processing just change the plugin version:
+
+```kotlin
+// build.gradle.kts
+plugins {
+    id("org.processing.java") version "4.5.0"
+}
+```
+
+Other gradle plugins are also supported
+
+```kotlin
+// build.gradle.kts
+plugins {
+    id("org.processing.java") version "4.5.3"
+    id("com.gradleup.shadow") version "<version>"
+}
+```
+
+If you want to combine multiple sketches into a single project
+
+```kotlin
+// sketch-a/build.gradle.kts
+plugins {
+    id("org.processing.java") version "4.5.3"
+}
+
+// sketch-b/build.gradle.kts
+plugins {
+    id("org.processing.java") version "4.5.3"
+}
+
+// build.gradle.kts
+plugins {
+    id("org.processing.java") version "4.5.3" apply false
+}
+// settings.gradle.kts - create the file but leave blank
+```
+
+Then run all sketches at once with `gradle sketch`