added documentation

README.md

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+## Overview
+Envelope is a simple Gradle plugin that allows you to create an executable jar file
+that includes all runtime dependencies and can be executed with a simple
+
+```bash
+java -jar my-app.jar
+```
+It supports JPMS, embedded system properties, Java agents, extra folders to be added to classpath. 
+
+### Usage
+
+Declare the plugin in your build's `settings.gradle` like this
+```groovy
+
+pluginManagement {
+    repositories {
+        maven {
+            url = 'https://woggioni.net/mvn/'
+        }
+    }
+
+    plugins {
+        id "net.woggioni.gradle.envelope" version "2023.09.25"
+    }
+}
+```
+
+Then add it to a project's `build.gradle`
+
+```groovy
+plugins {
+    id 'net.woggioni.gradle.envelope'
+}
+
+envelopeJar {
+    mainClass = 'your.main.Class'
+}
+```
+
+The plugin adds 2 tasks to your project:
+
+- `envelopeJar` of type `net.woggioni.gradle.envelope.EnvelopeJarTask` that creates the executable jar in the project's libraries folder
+- `envelopeRun` of type `org.gradle.api.tasks.JavaExec` which launches the jar created by the `envelopeJar` task
+
+### Configuration
+
+`EnvelopeJarTask` has several properties useful for configuration purposes:
+
+###### mainClass 
+
+This string property sets the class that will be searched for the `main` method to start the application
+
+###### mainModule
+
+When this string property is set, the jar file will be started in JPMS mode (if running on Java 9+) and 
+this module will be searched for the main class, if the `mainClass` is not set the main class specified 
+in the module descriptor will be loaded instead
+
+###### systemProperties
+
+This is a map that contains Java system properties that will be set before your application starts
+
+###### extraClasspath
+
+This is a list of strings representing filesystem paths that will be added to the classpath (if running in classpath mode) 
+or to the module path (if running in JPMS mode) when the application starts. 
+
+Relative paths and interpolation with Java System properties and environmental variables are supported:
+
+e.g.
+
+This looks for a `plugin` folder in the user's home directory
+```
+${env:HOME}/plugins
+```
+
+Same using Java system properties instead
+```
+${sys:user.home}/plugins
+```
+
+###### javaAgent
+This is a method accepting 2 strings, the first is the Java agent classname and the second one is the java agent arguments.
+It can be invoked multiple times to setup multiple java agents for the same JAR file. 
+All the java agents will be invoked before the application startup.
+
+### Example
+
+```groovy
+plugins {
+    id 'net.woggioni.gradle.envelope'
+}
+
+envelopeJar {
+    mainClass = 'your.main.Class'
+    mainModule = 'your.main.module'
+
+    systemProperties = [
+        'some.property' : 'Some value'
+    ]
+
+    extraClasspath = ["plugins"]
+    
+    javaAgent('your.java.agent.Class', 'optional agent arguments')
+}
+```
+
+### Limitations
+
+This plugin requires Gradle >= 6.0 and Java >=0 8 to build the executable jar file.
+The assembled envelope jar requires and Java >= 8 to run, if only `mainClass` is specified,
+if both `mainModule` and `mainClass` are specified the generated jar file will (try to) run in classpath mode on Java 8
+and in JPMS mode on Java > 8.