Add README.md to export maven/gradle output with build/run instructions

[quintesse]

Just what the title says, it would be nice if the export gradle/maven command would add a README that explains how to build and run the project. An introduction saying it was generated using JBang's export command (with a link to the JBang website) would be nice too. Perhaps even add a link to the original file if it was a URL.

Something like:

   # <script-name-here>

   This project was generated using the [JBang](www.jbang.dev) export command.

   ## Building

   This assumes you have an appropriate JDK installed on your PATH. You can install JDKs using
   JBang's [`jdk` command](https://www.jbang.dev/documentation/guide/latest/javaversions.html#managing-jdks).

   '''
   mvnw clean package
   '''

   ## Running

   '''
   java -jar target/<script-name-here>-999-SNAPSHOT.jar
   '''