updated readme.adoc with gemPath and requires parmeters for issue #122

This PR only includes a small correction on the README.adoc to include:
* `gemPath` and `requires` descriptions
* New section linking to the asciidoctor-maven-examples project

Note: after playing a bit with gemPath parameter I think leaving as is it is the best solution. Converting it to a list adds extra complexity in pom which only helps in a very few use cases.