Provide an example of adding a JAR to the classpath of LemMinX

[datho7561]

In #336 we will add the ability to add LemMinX extensions to the LemMinX classpath through a setting. Provide an example of adding a LemMinX extension to demonstrate how this works.

Specifically, provide an example in Preferences.md

[rgrunber]

https://github.com/redhat-developer/vscode-xml/blob/master/docs/Preferences.md#extension-jars -> https://github.com/redhat-developer/vscode-xml/blob/master/docs/Extensions.md#custom-xml-extensions and one of the examples mentions how to use xml.extension.jars.