Add a hint how to create a minified, runnable version of the example

[julicjung]

Briefly explain as part of the guide how build.gradle needs to be configured to produce a runnable jar file. I feel a runnable jar file is something many people might be looking for in a getting started guide, e.g. if they come from a SpringBoot background.

I found out how to do it by searching around the documentation and piecing things together after looking at the server CLI. It might be beneficial to explain how it is done by including a hint for where and how this is configured.
liberty { server { [omitted] packageLiberty { archive = "$buildDir/${appName}.jar" include = "minify,runnable" } } }